Overcurrent/overpower protection method of switching power supply, protection circuit and switching power supply

The invention discloses an overcurrent/overpower protection method of a switching power supply. The protection method is applied to the switching power supply. The switching power supply comprises a control circuit used for controlling breakover and cut-off of a power tube, wherein the control circuit comprises an overcurrent/overpower protection module. The protection method comprises the following steps: a first signal is collected and kept to form a line voltage compensation signal which is constant in a preset time range, and the line voltage compensation signal is loaded to the overcurrent/overpower protection module, and the first signal is matched with the duty cycle of a PWM (Pulse-Width Modulation) signal, so that a line voltage compensation value is kept constant in a certain time range by the method, and is not timely changing along with the PWM signal, so that the sub-harmonic oscillation is effectively restrained from the source.