A control circuit and a method for controlling a power converter are provided. The method for controlling the power converter includes the following steps. A detection signal is received from the secondary side of the power transformer and a first switching signal is generated in accordance with the detection signal. A second switching signal is generated in accordance with the first switching signal. A voltage signal is generated in accordance with the second switching signal. A comparison signal is generated in accordance with the first switching signal and the second switching signal. The voltage signal and the comparison signal are compared for outputting a comparison result. A gate signal is generated in accordance with the detection signal and the comparison result to control on and off states of a synchronization switch.