A controlling circuit is provided for controlling an output voltage of a main circuit of a switched-mode power supply. The controlling circuit includes components that generate a switching status selecting signal reflecting a voltage change of the main circuit, and output a reference voltage according to a voltage output status selected according to the switching status selecting signal. Other components output a pulse width modulation controlling signal according to the reference voltage and a current signal reflecting a current change of the main circuit, and output a pulse frequency modulation controlling signal with a frequency according to a frequency output status selected according to the switching status selecting signal. Yet other components output a switching controlling signal according to the controlling signals, and control a switch of the main circuit to switch-on or switch off according to the switching controlling signal to stabilize the output voltage of the main circuit.