A switching power supply system includes a switching converter, to convert an input voltage into an output voltage and to generate a switching signal; a feedback circuit, to generate a feedback signal; an error amplifier to generate an error signal; a triangle signal generator to generate a triangle signal; a constant on time control circuit to receive error signal and the triangle signal, and to generate a constant on time control signal to control power switch; in the system. The triangle signal has a DC bias based on either a soft start signal or a second reference signal. The system could perform soft start function and meanwhile keep matching between the error signal and the triangle signal.