Thanks for the replies again.
Maybe I didn't express clearly what I really want to the salesperson,so he handed me a switch video device....Worse still,I have no idea what KVM is all about because I only use SSH to connect to my server before and didn't really care about the cost of buying additional keyboards,mouse and monitor.
odevans,thanks for your big picture.I know what I exactly want.
However,it costs much.I think about $22 U.S. dollars for the little box.Anyway,I will check this out.