Hi, I am US citizen and currently married with an US citizen. Both of us living in USA. Now If I go to south america and mary another person there at south america while I am still married here in USA with US citizen.
Will my citizenship in USA get canceled? What can happen? Please answer this situation.

You will be come a criminal in the US. If you return to the US you could be arrested and put in jail, since it is not legal to be married to two people at the same time.

By US law, your second marriage would not be legal. Also I don't know any south american nations that allow you to have two wives, so most likely you can not legally marry there without getting a divorce in US first.