SOURCE

var app = new Vue({
  el:'#app',
  data() {
    return {
      form:{
        aaa:"123456"
      }
    }
  },
  methods:{
    setStr(){
      this.form.aaa="string";
      //this.$set(this.form,"aaa","string")
      console.log(this.form.aaa);
    }
  }
})

<div id="app">
    <el-input
      placeholder="请输入内容"
      v-model="form.aaa"
      >
    </el-input>
<div style="margin: 20px 0;">
  <button @click="setStr()">按钮</button>
</div>
</div>