<script type ="text/javascript" language="javascript">
           function checkInput() {        
          var strMsg = "";
          var userName = document.getElementById("<%=txt_Uer.ClientID%>").value;
          var password = document.getElementById("<%=txt_Password.ClientID%>").value;
          var txt_key = document.getElementById("<%=txt_key.ClientID%>").value;       
          if (userName == "" || userName == null) {
              strMsg = "用户名"
          }
          if (password == "" || password == null) {
              strMsg += " 密码"
          }
          if (txt_key == "" || txt_key == null) {
              strMsg += " 验证码"
          }
          if (strMsg != "") {
              alert(strMsg + " 不能为空！");
              return false;
          }
      }
    </script>

   <asp:Button ID="Button1" runat="server" Text="登录" onclick="Button1_Click" OnClientClick="return checkInput()" />