前台代码：
 <asp:Button ID ="addPic" runat ="server" Text ='添加附件' onclick="addPic_Click" />
        <div  style ="display :none;">        
        <asp:FileUpload ID="fuPhoto"  onchange="javascript:__doPostBack('lbUploadPhoto','')" 
                                        runat="server" ToolTip="选择图片" /> 
        </div>

点击按钮事件：
 protected void addPic_Click(object sender, EventArgs e)
        {
            ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "ListErrorTip", "document.getElementById('fuPhoto').click();", true);

        }

我之所以想在后台点击控件是因为我想在点击事件里面先做一些检查，检查通过了才弹出窗口。抱歉没有园豆，知道的回答下