diff --git a/examples/debug.tsx b/examples/debug.tsx
index 9a447e5..7aa8685 100644
--- a/examples/debug.tsx
+++ b/examples/debug.tsx
@@ -7,7 +7,7 @@ import '../assets/index.less';
 const { Option } = Mentions;
 
 export default () => (
-  <Mentions rows={3} defaultValue="Hello World" open>
+  <Mentions rows={3} defaultValue="Hello @ World @" open>
     <Option value="light">Light</Option>
     <Option value="bamboo">Bamboo</Option>
     <Option value="cat">Cat</Option>
diff --git a/src/Mentions.tsx b/src/Mentions.tsx
index 98565ee..28f083d 100644
--- a/src/Mentions.tsx
+++ b/src/Mentions.tsx
@@ -2,6 +2,7 @@ import classNames from 'classnames';
 import useMergedState from 'rc-util/lib/hooks/useMergedState';
 import toArray from 'rc-util/lib/Children/toArray';
 import KeyCode from 'rc-util/lib/KeyCode';
+import warning from 'rc-util/lib/warning';
 import * as React from 'react';
 import { useState, useRef } from 'react';
 import TextArea from 'rc-textarea';
@@ -75,6 +76,8 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
     defaultValue,
     children,
 
+    open,
+
     // Events
     validateSearch,
     filterOption,
@@ -99,6 +102,12 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
     ...restProps
   } = props;
 
+  const mergedPrefix = Array.isArray(prefix) ? prefix : [prefix];
+  const mergedProps = {
+    ...props,
+    prefix: mergedPrefix,
+  };
+
   // =============================== Refs ===============================
   const textareaRef = useRef<TextArea>(null);
   const measureRef = useRef<HTMLDivElement>(null);
@@ -122,6 +131,48 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
     value: value,
   });
 
+  // =============================== Open ===============================
+  const [
+    mergedMeasuring,
+    mergedMeasureText,
+    mergedMeasurePrefix,
+    mergedMeasureLocation,
+  ] = React.useMemo<
+    [
+      typeof measuring,
+      typeof measureText,
+      typeof measurePrefix,
+      typeof measureLocation,
+    ]
+  >(() => {
+    if (open) {
+      if (process.env.NODE_ENV !== 'production') {
+        warning(
+          false,
+          '`open` of Mentions is only used for debug usage. Do not use in you production.',
+        );
+      }
+
+      for (let i = 0; i < mergedPrefix.length; i += 1) {
+        const curPrefix = mergedPrefix[i];
+        const index = mergedValue.lastIndexOf(curPrefix);
+        if (index >= 0) {
+          return [true, '', curPrefix, index];
+        }
+      }
+    }
+
+    return [measuring, measureText, measurePrefix, measureLocation];
+  }, [
+    open,
+    measuring,
+    mergedPrefix,
+    mergedValue,
+    measureText,
+    measurePrefix,
+    measureLocation,
+  ]);
+
   // ============================== Option ==============================
   const getOptions = React.useCallback(
     (targetMeasureText: string) => {
@@ -151,8 +202,8 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
   );
 
   const options = React.useMemo(
-    () => getOptions(measureText),
-    [getOptions, measureText],
+    () => getOptions(mergedMeasureText),
+    [getOptions, mergedMeasureText],
   );
 
   // ============================= Measure ==============================
@@ -195,9 +246,9 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
 
     const { value: mentionValue = '' } = option;
     const { text, selectionLocation } = replaceWithMeasure(mergedValue, {
-      measureLocation,
+      measureLocation: mergedMeasureLocation,
       targetText: mentionValue,
-      prefix: measurePrefix,
+      prefix: mergedMeasurePrefix,
       selectionStart: getTextArea()?.selectionStart,
       split,
     });
@@ -207,7 +258,7 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
       setInputSelection(getTextArea(), selectionLocation);
     });
 
-    onSelect?.(option, measurePrefix);
+    onSelect?.(option, mergedMeasurePrefix);
   };
 
   // ============================= KeyEvent =============================
@@ -220,7 +271,7 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
     onKeyDown?.(event);
 
     // Skip if not measuring
-    if (!measuring) {
+    if (!mergedMeasuring) {
       return;
     }
 
@@ -264,7 +315,7 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
     const target = event.target as HTMLTextAreaElement;
     const selectionStartText = getBeforeSelectionText(target);
     const { location: measureIndex, prefix: nextMeasurePrefix } =
-      getLastMeasureIndex(selectionStartText, prefix);
+      getLastMeasureIndex(selectionStartText, mergedPrefix);
 
     // If the client implements an onKeyUp handler, call it
     onKeyUp?.(event);
@@ -281,19 +332,22 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
       const nextMeasureText = selectionStartText.slice(
         measureIndex + nextMeasurePrefix.length,
       );
-      const validateMeasure: boolean = validateSearch(nextMeasureText, props);
+      const validateMeasure: boolean = validateSearch(
+        nextMeasureText,
+        mergedProps,
+      );
       const matchOption = !!getOptions(nextMeasureText).length;
 
       if (validateMeasure) {
         if (
           key === nextMeasurePrefix ||
           key === 'Shift' ||
-          measuring ||
-          (nextMeasureText !== measureText && matchOption)
+          mergedMeasuring ||
+          (nextMeasureText !== mergedMeasureText && matchOption)
         ) {
           startMeasure(nextMeasureText, nextMeasurePrefix, measureIndex);
         }
-      } else if (measuring) {
+      } else if (mergedMeasuring) {
         // Stop if measureText is invalidate
         stopMeasure();
       }
@@ -305,7 +359,7 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
       if (onSearch && validateMeasure) {
         onSearch(nextMeasureText, nextMeasurePrefix);
       }
-    } else if (measuring) {
+    } else if (mergedMeasuring) {
       stopMeasure();
     }
   };
@@ -313,7 +367,7 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
   const onInternalPressEnter: React.KeyboardEventHandler<
     HTMLTextAreaElement
   > = event => {
-    if (!measuring && onPressEnter) {
+    if (!mergedMeasuring && onPressEnter) {
       onPressEnter(event);
     }
   };
@@ -359,9 +413,9 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
         onFocus={onInternalFocus}
         onBlur={onInternalBlur}
       />
-      {measuring && (
+      {mergedMeasuring && (
         <div ref={measureRef} className={`${prefixCls}-measure`}>
-          {mergedValue.slice(0, measureLocation)}
+          {mergedValue.slice(0, mergedMeasureLocation)}
           <MentionsContext.Provider
             value={{
               notFoundContent,
@@ -382,10 +436,12 @@ const Mentions = React.forwardRef<MentionsRef, MentionsProps>((props, ref) => {
               getPopupContainer={getPopupContainer}
               dropdownClassName={dropdownClassName}
             >
-              <span>{measurePrefix}</span>
+              <span>{mergedMeasurePrefix}</span>
             </KeywordTrigger>
           </MentionsContext.Provider>
-          {mergedValue.slice(measureLocation + measurePrefix.length)}
+          {mergedValue.slice(
+            mergedMeasureLocation + mergedMeasurePrefix.length,
+          )}
         </div>
       )}
     </div>
diff --git a/src/util.ts b/src/util.ts
index 7170b68..75d6295 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -9,10 +9,6 @@ export function getBeforeSelectionText(input: HTMLTextAreaElement) {
   return input.value.slice(0, selectionStart);
 }
 
-export function toArr<T>(value: T | T[]): T[] {
-  return Array.isArray(value) ? value : [value];
-}
-
 interface MeasureIndex {
   location: number;
   prefix: string;
@@ -22,10 +18,9 @@ interface MeasureIndex {
  */
 export function getLastMeasureIndex(
   text: string,
-  prefix: string | string[] = '',
+  prefix: string[],
 ): MeasureIndex {
-  const prefixList: string[] = toArr(prefix);
-  return prefixList.reduce(
+  return prefix.reduce(
     (lastMatch: MeasureIndex, prefixStr): MeasureIndex => {
       const lastIndex = text.lastIndexOf(prefixStr);
       if (lastIndex > lastMatch.location) {
diff --git a/tests/Open.spec.tsx b/tests/Open.spec.tsx
new file mode 100644
index 0000000..43f6595
--- /dev/null
+++ b/tests/Open.spec.tsx
@@ -0,0 +1,27 @@
+import React from 'react';
+import Mentions from '../src';
+import { expectMeasuring } from './util';
+import { render } from '@testing-library/react';
+
+const { Option } = Mentions;
+
+describe('Mentions.Open', () => {
+  it('force open', () => {
+    const errorSpy = jest.spyOn(console, 'error').mockImplementation(() => {});
+
+    const { container } = render(
+      <Mentions open defaultValue="@cat @">
+        <Option value="bamboo">Bamboo</Option>
+        <Option value="light">Light</Option>
+        <Option value="cat">Cat</Option>
+      </Mentions>,
+    );
+
+    expectMeasuring(container);
+
+    expect(errorSpy).toHaveBeenCalledWith(
+      'Warning: `open` of Mentions is only used for debug usage. Do not use in you production.',
+    );
+    errorSpy.mockRestore();
+  });
+});