It is reported firstly that the plasma extraction transit time(PETT) oscillations occur under short circuit condition of high-power IGBT modules. First, a test platform with high-bandwidth test equipment is constructed to evaluate PETT oscillations under different short-circuit conditions, and the oscillation mechanism is characterized based on the carrier and electric field distribution inside the drift region during the short circuit turn-off process. Moreover, the effects of bus voltage, gate voltage(short current), junction temperature and gate resistance on PETT oscillations under short circuit conditions are analyzed based on experimental results. Finally, a method with package layout optimization to suppress PETT oscillations is proposed and the effectiveness of proposed method is verified by experiment.