function a()
{
document.getElementById("btnok").value = '正在提交
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document.getElementById("btnok").onclick=function(){return false;};
return true;
}
<input id="btnok" runat="server" type="submit" value="确定" onclick="return a();"
onserverclick="Submit1_ServerClick" />
//方法二:必需要是 Asp.net服务器控件。(注意UseSubmitBehavior属性)
<asp:Button ID="btnSumbit" runat="server" UseSubmitBehavior="false"
OnClientClick="this.value='正在提交
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Text="提交" OnClick="btnSumbit_Click" />