<el-form-item label="藕粉桂花糖糕" prop="state" required>
          <el-select
            ref="mySelect"
            v-model="form.state"
            style="width: 280px"
            placeholder="请选择"
          >
            <el-option label="藕粉" :value="1" />
            <el-option label="桂花" :value="2" />
            <el-option label="糖糕" :value="3" />
          </el-select>
</el-form-item>

解决办法：使用 toggleMenu() 方法 

created() {
    // 用之前得 this.$nextTick，需要 dom 存在才好使
    this.$nextTick(() => {
      this.$refs.mySelect.toggleMenu()
    })
},