The present invention provides a controller used in a switching power supply. The controller comprises an oscillator, a first comparison circuit and a logic circuit. The oscillator generates a slope compensation signal and a clock signal. The first comparison circuit generates a first comparison signal in accordance with a current sensing signal, a feedback signal and the slope compensation signal. The logic circuit generates a control signal to control the main switch based on the clock signal and the first compensation signal. If the on time of the main switch is longer than the predetermined time period, the slope compensation signal will have a first slew rate during the predetermined time period and a second slew rate out of the predetermined time period, wherein the second slew rate is smaller than the first slew rate.