watch:{
    listQuery: {
        handler(oldval,newval){
            if(newval.customerName===''||newval.customerName===undefined){
                this.listQuery.matter = ''
            }
        },
        deep:true
    }
  }

<el-form-item label="單位" prop="customerName" label-width="58px">
          <el-select v-model="listQuery.customerName" filterable style="width: 160px"
                     clearable placeholder
 
="請選擇單位名稱" >
            <el-option
              v-for="item in customerList"
              :key="item.id"
              :label="item.name"
              :value="item.name">
            </el-option>
          </el-select>
        </el-form-item>

        <el-form-item label="物料" prop="matter" label-width="58px">
          <el-select v-model="listQuery.matter" filterable style="width: 160px"
                     clearable placeholder
 
="請選擇物料" :disabled="listQuery.customerName===''||listQuery.customerName===undefined">
            <el-option
              v-for="item in matterList"
              :key="item.id"
              :label="item.name"
              :value="item.name">
            </el-option>
          </el-select>
        </el-form-item>