A power safety system includes a first MOS, a second MOS, a switch and a body controller. The first MOS is connected between a power input and a power output. The second MOSFET is connected between the power output and a charging output. The switch has an end connected to the body of the first MOS, and the opposite end switched between the source and the drain of the first MOS. A body controller controls the switch according to the voltage at the power input and the voltage at the power output, to connect the body of the first MOS to the source or the drain of the first MOS. By switching the switch, the first MOS will have a parasitic diode effective to prevent a reverse current from the power output to the power input.