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In the original js thinking, the selection of cascading drop-down boxes is often to first bind the menu of the first-level drop-down box, then onchange in the drop-down box, get the current option in the onchange event, and then bind the data of the second-level drop-down box, and so on...

In the Vue framework, you should change your thinking. First, set the data of the first-level drop-down box, then watch the option. If it changes, set the data of the second-level drop-down box, and so on:

<el-form-item label="省">
      <el-select v-model="where.provinceId" placeholder="请选择省份" clearable >
             <el-option v-for="item in provinces" :key="item.id" :label="item.name" :value="item.id" />
       </el-select>
</el-form-item>
<el-form-item label="市">
      <el-select v-model="where.cityId" placeholder="请选择市" clearable>
             <el-option v-for="item in cities" :key="item.id" :label="item.name" :value="item.id" />
       </el-select>
</el-form-item>

const provinces = ref([])
const cities = ref([])
 
onMounted(()=>{
    // 通过接口获取省份
    provinces.value = [...data]
})
 
watch: {
    provinceId: {
      deep: true,
      handler() {
        // 根据接口获取市的数据
        cities.value=[...data]
        // 清空城市的选择
        where.cityId=''
      }
    }
},