A polarity switching circuit includes: a first current-limiting resistor and a second current-limiting resistor connected to a DC high voltage; a first transistor switch, a second transistor switch, a fourth transistor switch, and a fifth transistor switch respectively controlled by a first PWM signal and a second PWM signal; a third transistor and a sixth transistor switch whose control terminals are respectively connected to the first transistor switch and the fourth transistor switch; a first filter connected to the second transistor switch and the third transistor switch and a contact of a piezoelectric actuator; and a second filter connected to the fifth transistor switch and the sixth transistor switch and another contact of the piezoelectric actuator. When the first and the second PWM signal are switching between a high level and a low level, output AC voltages with smoothed AC waveforms are supplied to the contacts of the piezoelectric actuator.