代码：
<form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <div>
                    <asp:TextBox ID="TextBox1" runat="server" AutoPostBack="true" OnTextChanged="TextBox1_TextChanged"></asp:TextBox>
                    <asp:TextBox ID="TextBox2" runat="server" TabIndex="9"></asp:TextBox>
                    <asp:TextBox ID="TextBox3" runat="server" TabIndex="10"></asp:TextBox>
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>
    </form>
后台：
protected void TextBox1_TextChanged(object sender, EventArgs e)
    {
        TextBox2.Text = DateTime.Now.ToString();
        ScriptManager.RegisterClientScriptBlock(this.UpdatePanel1, this.GetType(), "show1","document.getElementById('TextBox3').focus();", true);
    }
这样写TextBox1输入内容改变后，焦点到不了TextBox3，但是在document.getElementById('TextBox3').focus();加个alert('1');又可以。在网上查到focus方法延迟触发，放在setTimeout中也不行；就是setTimeout(function() {document.getElementById('TextBox3').focus();},200); 同样在前面加个alert又可以