核心关键是：ClientInstanceName="popup" CloseAction="None" ShowHeader="False" Modal="True" ，否则，弹窗消失甚至不再显示
<dx:ASPxPopupControl ID="ASPxPopupControl_tytsxx" runat="server" EnableViewState="False" HeaderText="单击右上角的✘关闭" PopupHorizontalAlign="WindowCenter" PopupVerticalAlign="WindowCenter" ClientInstanceName="popup" CloseAction="None" ShowHeader="False" Modal="True">
<ContentCollection>
<dx:PopupControlContentControl runat="server">
<dx:ASPxRoundPanel ID="ASPxRoundPanel2" runat="server" ShowHeader="False" Width="250px">
<PanelCollection>
<dx:PanelContent runat="server">
<table align="center" width="240px">
<tr>
<td class="auto-style7" style="text-align:left">
<dx:ASPxLabel ID="ASPxLabel_tytsxx" runat="server" Theme="Material">
</dx:ASPxLabel>
</td>
</tr>
<tr style="height:40px">
<td>
<center>
<dx:ASPxButton ID="ASPxButton_tcgb" runat="server" Text="关闭">
<ClientSideEvents Click="function(s, e) {popup.Hide(); return false;}" />
</dx:ASPxButton>
</center>
</td>
</tr>
</table>
</dx:PanelContent>
</PanelCollection>
</dx:ASPxRoundPanel>
</dx:PopupControlContentControl>
</ContentCollection>
</dx:ASPxPopupControl>
代码：
if (cx_xkxx.Count() > 0)
{
//累积每一个年度的成绩
nr =nj + bj + "班 " + xm + " 选修课成绩单" + "\n\n";
foreach (var kk in cx_xkxx)
{
nr +=kk.Bynf + kk.Xq + "【" + kk.Kcmc + "】" + ((int)kk.fs).ToString() + "分\n";
}
// aspnetajaxjavascript.jqtx(this.UpdatePanel1, "操作提示", nr, "480px", "400px");
this.ASPxLabel_tytsxx.Text = nr;
this.ASPxPopupControl_tytsxx.ShowOnPageLoad = true;
}
else
{
// aspnetajaxjavascript.jqtx(this.UpdatePanel1, "操作提示", "没有查询到你的选课成绩", "480px", "400px");
this.ASPxLabel_tytsxx.Text = "没有查询到你的选课成绩";
this.ASPxPopupControl_tytsxx.ShowOnPageLoad = true;
}