<input type="text" id="txt">

//字符串截取
function getByteVal(val, max) {
 var returnValue = '';
 var byteValLen = 0;
 for (var i = 0; i < val.length; i++) {
  if (val[i].match(/[^\x00-\xff]/ig) != null)
  byteValLen += 2;
  else
  byteValLen += 1;
  if (byteValLen > max)
  break;
  returnValue += val[i];
 }
 return returnValue;
}
$('#txt').bind('keyup',function(){
 var val=this.value;
 if(val.replace(/[^\x00-\xff]/g,"**").length>14){
  this.value=getByteVal(val,14)
 }
})