Method for restraining starting-up impulse current of switching power supply

The invention discloses a method for restraining starting-up impulse current of a switching power supply. According to the method, when alternating voltage is connected, the instant waveform position of the input alternating voltage is firstly detected, and when it is detected that the waveform of the alternating voltage is located within 2.5 ms before zero volt, a comparator outputs high level to drive a relay to be pulled in, and alternating current starts to be supplied for the rear-end switching power supply. After the rear-end switching power supply is started, 12V boosting voltage is provided to maintain pull-in of the relay, and after pull-in of the relay is delayed by a certain period through an internal charging capacitor, a normally-closed input end relay is made to trip, and the part of circuit is shut off to save energy. The instant peak amplitude of input current is small and can be limited below a normal full-load current value.