前台
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<br />
<br />
<br />
<br />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button ID="Button1" runat="server" Text="测试" onclick="Button1_Click" />
</ContentTemplate>
</asp:UpdatePanel>
</div>
</form>
</body>
后台
protected void Button1_Click(object sender, EventArgs e)
{
string js = "$get('TextBox1').value='测试';";
ScriptManager.RegisterStartupScript(Button1, GetType(), "", js, true);
}