试试 value 属性，像这样:

Try the value property, like this:

var fu1 = document.getElementById("FileUpload1");
alert("You selected " + fu1.value);

注意:看起来 FileUpload1 是一个 ASP.Net 服务器端 FileUpload 控件.
如果是这样，您应该使用 ClientID 属性，像这样:

NOTE: It looks like FileUpload1 is an ASP.Net server-side FileUpload control.
If so, you should get its ID using the ClientID property, like this:

var fu1 = document.getElementById("<%= FileUpload1.ClientID %>");