A power supply device and an overvoltage protection method are disclosed. The power supply device includes an energy storage unit, a first switch, a second switch and a driving module. The energy storage unit generates a driving voltage according to a supplied voltage. The first switch selectively transmits the supplied voltage to the energy storage unit according to a first driving signal. The second switch is selectively conducted to adjust the driving voltage according to a second driving signal. The driving module conducts the second switch when the supplied voltage is rose at a first predetermined value, and generates the first driving signal and the second driving signal after the supplied voltage is rose at a second predetermined value. The first predetermined value is smaller than the second predetermined value.