The present invention discloses a constant on-time switching regulator, a control method therefor, and an on-time calculation circuit for calculating an on-time period of a constant on-time switching regulator. The on-time calculation circuit calculates on-time according to practical conditions. It includes: a driver gate receiving a gate signal of a power switch in a switching regulator, the driver gate operating between high and low levels of a first reference voltage and ground; a low pass filter receiving an output from the driver gate and generating a second reference voltage, a ratio between the second reference voltage and the first reference voltage being substantially the same as a duty ratio of the gate signal; and an on-time generator comparing the second reference voltage with a ramp signal to determine an on-time of the power switch.