<script language="javascript" type="text/javascript">      

  function textLimitCheck(thisArea, maxLength, SpanId)      

  {

  if (thisArea.value.length > maxLength)          

  {               

  thisArea.value = thisArea.value.substring(0, maxLength);              

  thisArea.focus();          

  }          

  /*回写span的值，当前填写文字*/ 

  var varss='（剩余字数：'+(maxLength-thisArea.value.length)+'）';     

  document.getElementById(SpanId).innerHTML =varss;      

  }  

  </script>

<asp:TextBox ID="txtOwner_Name" runat="server" Width="250px" MaxLength="50"  onkeyup="textLimitCheck(this,50,'spOwner_Name')" BackColor="Info"></asp:TextBox>

<span id="spOwner_Name" style="color: gray"><em>（50字以内）</em></span>