A square wave generator includes a sawtooth wave generator for generating a sawtooth wave, and a convertor for generating a square wave based on the sawtooth wave. The sawtooth wave generator includes a capacitor and a switching unit connected parallel to each other. A first terminal of the capacitor is electrically coupled to a power source and the convertor, and a second terminal of the capacitor is grounded. The switching unit includes a trigger and a field-effect transistor. When a voltage of the first terminal of the capacitor is not less than a first threshold voltage of the trigger, the trigger is turned on to activate the field-effect transistor, the field-effect transistor is turned on to ground the first terminal of the capacitor, so that the capacitor discharges rapidly.