我怀疑val[0].Id是没有值的
打印两条看
console.log(val[0].Id)
console.log(val)

handleSelectionChange这个方法的参数val，指的是原本的数据，一定要注意要深拷贝，不然这里直接修改val，原本的下拉菜单的值就没有了。

参考代码：

      <el-table
        ref="multipleTable"
        :data="gridData"
        tooltip-effect="dark"
        style="width: 100%"
        @selection-change="handleSelectionChange"
      >
        <el-table-column
          type="selection"
          style="margin-left: 10px"
        />
      </el-table>
 
 
    handleSelectionChange(val) {
      this.$forceUpdate()
      const newValue = JSON.parse(JSON.stringify(val))
      this.multipleSelection = newValue
      for (let i = 0; i < this.multipleSelection.length; i++) {
        this.multipleSelection[i].database = this.selectMultipleValue[this.multipleSelection[i].businessLine + this.multipleSelection[i].sourceType]
      }
      this.$forceUpdate()
    },

 handleSelectionChange(val) {

      this.$forceUpdate()
      const newValue = JSON.parse(JSON.stringify(val))
      this.ids=newValue

      if(this.ids.indexOf(val[0].Id) < 0 ){
        this.ids.push(val[0].Id)
      } else {
        this.ids.splice(this.ids.indexOf(val[0].Id), 1)
      }
      console.log(idddd,this.Id)
      console.log(val)
      //  console.log('xxx',this.ids)
    },