im wondering if i can make a group of control that will stay on screen even someone on another webpage while my webpage open in another tab,in asp.net.Like winform's topmost feature.
No, you can not do this.
If you're expecting to port a winforms application to webforms feature for feature, you're going to be greatly disappointed. WebForms is still just a thin veneer over the http protocol; one that tends to peel up at the corner.
If you're trying to keep something on top via the web, you can't do that without a plugin, no matter what web platform you use.