廢話不多說啦，直接上demo：

<template>
    <span class="circle"></span><span class="text">限制{{remnant}}/600字以內</span>
    <textarea name="" id="" class="input_text" style="resize:none;" maxlength="600" @input = "descInput" v-model="desc">
    </textarea>
</template>
<script>
export default {
data() {
  return {
           remnant:600,//倒計數
         }
   }
},
methods: {
//倒計數
 descInput(){
   var txtVal = this.desc.length;
   this.remnant = 600-txtVal;
 },
}

</script>
 

 <el-form-item label="週記內容" prop="content">
        <el-input type="textarea" placeholder="請輸入內容" @input = "descInput" v-model="dataForm.content" style="width: 90%;" :rows="6" v-bind:readonly="infoStatus">
        </el-input><br>
        <span class="text" style="float: right;color: #909399;margin-right: 55px;">已輸入{{remnant}}字</span>
 </el-form-item>
<script>
export default {
data() {
  return {
           remnant:0,//輸入數
         }
   }
},
methods: {
//計算輸入數
 descInput(){
   var txtVal = this.desc.length;
   this.remnant = txtVal;
 },
}

</script>
 

效果如下：