A variable switching frequency power supply apparatus includes a main transformer, a voltage detection unit, a pulse width modulation control unit and a switch unit. The voltage detection unit is electrically connected to the main transformer. The pulse width modulation control unit is electrically connected to the voltage detection unit. The switch unit is electrically connected to the pulse width modulation control unit and the main transformer. The main transformer transforms an input voltage into an output voltage according to a switching frequency of the switch unit. The voltage detection unit detects the input voltage and then informs the pulse width modulation control unit. The pulse width modulation control unit is configured to control the switching frequency of the switch unit according to a voltage absolute value of the input voltage.