解决方法
HTML：
<asp:FileUpload ID="FileUpload1" onchange="selectFolder(event)" webkitdirectory mozdirectory msdirectory odirectory directory AllowMultiple="true" runat="server" />
要么
<input type="file" id="FileUpload" onchange="selectFolder(event)" webkitdirectory mozdirectory msdirectory odirectory directory multiple />
JavaScript的：
<script type="text/javascript">
function selectFolder(e) {
var theFiles = e.target.files;
var relativePath = theFiles[0].webkitRelativePath;
var folder = relativePath.split("/");
alert(folder[0]);
}
</script>
