An efficiency tracking method of a controller applied to a flyback power converter includes before the controller soft starts, an original frequency variation curve setting voltage generated by the controller outputting an original frequency variation curve setting detection current to an original frequency variation curve setting detection resistor determining a frequency variation curve of the flyback power converter, and utilizing adjustment of resistance of the original frequency variation curve setting detection resistor to achieve efficiency optimization. Therefore, the controller controls an output voltage of the flyback power converter and tracks a maximum power point of the flyback power converter according to the efficiency tracking method to achieve efficiency optimization.