A computer wake up circuit, for awaking a computer which is in a sleep state via a computer serial port, includes an AT89 series microcontroller, a switching circuit, an input circuit. The switching circuit is coupled to the pins P3.2 (INT0) and P3.7 of the AT89 series microcontroller, for triggering the AT89 series microcontroller to output an control signal. The input circuit comprises a first resistance, a control switch, a second resistance, and a third resistance. One end of the first resistance is coupled to pin P3.1 (TXD) of the AT89 series microcontroller, the other end of the first resistance is coupled to an input end of the control switch. An output end of the control switch is coupled to the second pin of the computer serial port via the second resistance, and is coupled to the third pin of the computer serial port via the third resistance.