官方是这样说的：

placeholder 只有在 value = undefined 才会显示，对于其它的 null、0、'' 等等对于 JS 语言都是有意义的值。

所以原本我的代码是这样的,这样写placeholder的值是不会显示的

<template>
<a-form-itemlabel="所属门店"name="shopId":rules="[{ required: true, message: '请输入所属门店' }]"
><a-selectv-model:value="formState.shopId":fieldNames="{ label: 'name', value: 'id' }":options="options"placeholder="请选择所属门店"style="width: 200px"></a-select>
</a-form-item>
</template><script lang='ts' setup>
const formState = reactive({shopId: "",model: 0,name: "",ylyTerminalNo: "",ylySecretKey: "",
});
</script>

解决办法：把shopId: " "改成shopId: undefined就行了

<template>
<a-form-itemlabel="所属门店"name="shopId":rules="[{ required: true, message: '请输入所属门店' }]"
><a-selectv-model:value="formState.shopId":fieldNames="{ label: 'name', value: 'id' }":options="options"placeholder="请选择所属门店"style="width: 200px"></a-select>
</a-form-item>
</template><script lang='ts' setup>
const formState = reactive({shopId: undefined,model: 0,name: "",ylyTerminalNo: "",ylySecretKey: "",
});
</script>