A protective device and method for preventing supply voltage of microcontroller from sag in electronic cigarette is provided, comprising a microcontroller, a power supply module, a field effect transistor, an energy storage circuit, wherein, the energy storage circuit is connected between the microcontroller and the power supply module, the energy storage circuit supplies electric power to the microcontroller when an over current or short circuit occurs, and maintains the supply voltage of the microcontroller not being less than its minimal operating voltage in a certain period of time; the microcontroller processes the over current or short circuit signal and turns off the MOSFET to cut off the current flow in a load circuit. The unstable or uncontrollable phenomena of the microcontroller in the existing technology are resolved. The circuit of the present invention is simple and low cost.