The invention discloses a quasi-resonant mode switching power supply controller and a control method thereof. The controller comprises a zero-crossing detection circuit which samples zero-crossing points and generates zero-crossing signals; a valley bottom conduction logic circuit which receives the zero-crossing signal and the maximum frequency signal, generates the valley bottom opening signal,and decides whether to select a new valley bottom for latching according to the preset judgment logic; a PWM logic control circuit which receives a valley bottom opening signal and a minimum frequencysignal, thereby controlling the PWM signal to be turned on, and controlling the PWM signal to be turned off according to the FB voltage signal and the CS voltage signal; and a time setting circuit which receives the PWM signal and the FB voltage signal, generates a maximum frequency signal and a minimum frequency signal, and thereby controlling the maximum frequency and the minimum frequency of the PWM operation. The invention not only can eliminate the audio noise caused by the continuous switching of adjacent valleys when the quasi-resonant mode operates, but also can improve the EMI, and has high use and popularization value.