The invention provides an active power factor correction system. Through arrangement of double protections, a first current protection is implemented when the current at a DC side exceeds a first current threshold, and a second current protection is implemented when the current exceeds a second current threshold. During the first current protection, the current at the DC side is not too high, when the voltage at an AC power supply passes zero next time, the output of a PWM pulse signal is restored; during the second current protection, the current at the DC side is high already, the output of the PWM pulse signal is stopped and then the PWM pulse signal is output again until a circuit is restored to be in a normal state, the PWM pulse signal output again is a newly adjusted PWM pulse signal, so that the circuit normally runs, and overcurrent is reduced. Current protection using software is more effective than protection using hardware. In this way, the possibility of PFC restarting can be reduced, and the circuit can run more stably and can be protected from being damaged due to overcurrent.