A control circuit and method for controlling a power converter comprises an adder, a digital compensator, and a pulse width modulation circuit. The adder acquires an output voltage difference between the output voltage and the reference output voltage. The digital compensator, which has a Z-domain transfer function, references the output voltage difference to generate a pulse width control signal with the least significant bits of the denominator coefficient in the Z-domain transfer function being regulated to achieve the load line function of the power converter. Further, the pulse modulation circuit is controlled by the pulse width control signal to generate the pulse width modulation signal to control ON/OFF of the power converter.