Abstract: In view of problem that serious voltage fluctuation underground may affect unit power factor of power supply, variable output DC voltage method was proposed to control DC side voltage of PWM rectifier, so as to ensure the PWM rectifier working with high power factor. Selection of DC side voltage and influence of fluctuating of AC side input voltage in a wide range on AC side input current and power factor were analyzed. Control principle and software implementation of variable output DC voltage method were introduced respectively. The test results show that the PWM rectifier with variable output DC voltage control method can work stably and always achieve high power factor under voltage fluctuations.