A portable device with an automatic power off protection and a method of achieving such a protection are related. The portable device circuit comprises a switch unit, a main body, and a battery. The main body includes an acceleration transducer that samples an analog acceleration signal; an analog-to-digital converter (ADC) that converts the sampled analog acceleration signal into a digital acceleration value; a memory that stores a critical acceleration value and an interrupt flag; and a micro-control unit (MCU) that compares the digital acceleration values with the critical acceleration value and the comparing result which may or may not change the value of the interrupt flag. When the acceleration of the portable device is greater than the critical acceleration and the interrupt flag is enable, the MCU sends a break signal to the switch unit to power off the portable device.