A power factor control circuit has a power factor controller that determines if the power factor control circuit is operated at a continuous current mode (CCM) or a discrete current mode (DCM), and outputs PWM signals corresponding to the present current mode. A duty cycle of the PWM signals is equal to a sum of a feed-forward control parameter and a current compensation parameter. The current compensation parameter contains a difference value between a reference current and an inductor current in the power factor control circuit. Accordingly, a switching power supply circuit can acquire the PWM signals corresponding to the present current mode to effectively resolve the issue of harmonic distortion.