将下面的方法注册到你的页面
function WebForm_FireDefaultButton(event, target) {
      var element = event.target || event.srcElement;
      if (!__defaultFired && event.keyCode == 13 && !(element && (element.tagName.toLowerCase() == "textarea"))) {

            var defaultButton;
      if (__nonMSDOMBrowser) {
            defaultButton = document.getElementById(target);
      }
      else {
            defaultButton = document.all[target];
      }
      if (defaultButton && typeof(defaultButton.click) != "undefined") {
            __defaultFired = true;
            defaultButton.click();
            event.cancelBubble = true;
            if (event.stopPropagation) 
                  event.stopPropagation();
            return false;
      }
      }      
      return true;
}