Abstract: A PFC switching power conversion circuit providing low THD, including: a bridge rectifier for providing an input voltage according to an AC voltage; an electromagnetic interference filtering circuit for providing a line voltage according to the input voltage; a switching power converter for converting the line voltage to an output voltage or current under a control of a driving voltage, the driving voltage causing a switching current to flow through a main inductor; and a controller for receiving and processing at least one feedback signal from the switching power converter to generate the driving voltage in a constant on-time manner, the controller including an off-time detector and a timer, the off-time detector being used to detect decreasing periods of the switching current to determine respective off-time periods, and the timer being used to set a minimum limit on the respective off-time periods.