A synchronous rectifier controller for an adaptive output power converter is provided. The synchronous rectifier controller includes a voltage detection circuit, a threshold generation circuit, and a driver. The voltage detection circuit detects an output voltage of the adaptive output power converter to generate a detection signal. The threshold generation circuit is coupled to the voltage detection circuit. The threshold generation circuit receives the detection signal and generates a synchronous rectifier (SR) turn-off threshold for a synchronous rectifier coupled to a secondary winding of the flyback converter according to the detection signal. The driver receives the SR turn-off threshold and controls the synchronous rectifier according to the SR turn-off threshold.