The present disclosure relates to a control circuit of the switching power supply including a soft-booting voltage generation circuit, a first comparator, a voltage selection circuit, a switching circuit, a second comparator, an error amplification circuit, and a pulse signal control circuit. During the soft-booting phase and during the operational phase, different superposition voltages are superposed with the control voltage by the voltage selection circuit to obtain the clamping voltage. Thus, different clamping voltages are configured during the soft-booting phase and the operational phase, which contributes to the system stability. In addition, when the output voltage of the switching circuit is greater than the reference voltage, the second comparator outputs the first high level signals to the clock signal generator. The clock signal generator then stops operations, that is, and the control circuit of the switching power supply is in the sleep mode, which enhances the efficiency of the power.