The present invention provides a switching regulator with PFC function and a control circuit and a control method thereof. The switching regulator with PFC function includes a power stage circuit, a current sense circuit, and a control circuit. The power stage circuit operates at least one power switch therein according to an operation signal to convert an input voltage to an output voltage. When a transient voltage of the input voltage exceeds a transient voltage upper limit, or when a transient slew rate of the input voltage exceeds a transient slew rate upper limit, the control circuit adjusts a frequency response gain from a stable state frequency response gain to a transient state frequency response gain, such that a transient current of an output current does not exceed a current upper limit, and/or that a transient response time of the output current does not exceed a threshold transient time period.