Why my Mac Pro with retina display is become hot while on charging
Because it is charging.
So it normal durning charging it becomes hot ...?
Yes. Don't cover the Mac with cases or "skins" or use keyboard covers, since they impede proper heat dissipation. Its entire case serves as a heat sink.
If you are concerned contact AppleCare. A Mac should never become so hot you can't hold it.
Retrieving data ...