解决办法就是UpdatePanel中设置PostBackTrigger：
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button1" runat="server" Text="上传" OnClick="Button1_Click" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="Button1" />
</Triggers>
</asp:UpdatePanel>