Disclosed are a control method and a control circuit for a switching power supply, said switching power supply comprises a secondary side controller and a secondary side MOS transistor connected between a load and a secondary side winding of a transformer. The present invention is used for detecting a working state of the secondary side winding of a transformer and a type of a communication signal transmitted by a load, and for generating a switching pulse signal VG in a Reset Time interval of an on/off cycle according to the type of the communication signal; the primary side controller detects a variation amplitude of the transiently varied signal of the voltage drop at the pin feedback (FB) in the Reset Time interval; if the variation amplitude of the transiently varied signal is greater than a pre-set value ΔVref, the primary side controller judges that the signal is a communication signal, and records the communication signal.