The invention provides a method for preventing power output currents from flowing backwards. The method comprises the following steps that a filtering and rectifying circuit is arranged at a current input end, alternating currents are converted into direct currents after being filtered and rectified through the filtering and rectifying circuit, and then the direct currents are converted into high voltage direct currents after being boosted through a PFC boosting circuit; the high voltage direct currents are converted into low voltage direct currents through a PWM control circuit and a transformer; the low voltage direct currents pass through an output rectifying and filtering circuit and then are output to be used by loads; a differentiating circuit is arranged between the PWM control circuit and the output rectifying and filtering circuit, and a clamping circuit is arranged among the output end of the PWM control circuit, the output end of the PFC boosting circuit and the input end of the transformer. According to the method for preventing the power output currents from flowing backwards, compared with the prior art, the currents are effectively prevented from flowing backwards, the converting efficiency of a power source is improved, and wasting of resources is avoided.