如下TextBox

<asp:textbox onkeypress="isnum()" id="TextBox1" runat="server"></asp:textbox>

1. 在页面中添加一段脚本：

<script language="javascript">
    function isnum()
    {
        if(event.keyCode<45 || event.keyCode>57)
        {
            event.keyCode=0;
        }
    }                             
</script>

2. 在代码中添加：

try
{
    Convert.ToDouble(TextBox1.Text.Trim());
}
catch
{
    Response.Write("<script>alert('请输入正确的数字格式！');</script>");
    TextBox1.Text="";
    Page.RegisterStartupScript("focus","<script>document.forms[0]['TextBox1'].focus();</script>");
}