A power-on circuit of a computer includes a heat sink, an SIO chip, a connector, a first electric switch, a second electric switch, and a third electric switch. When the heat sink is installed properly, the heat sink is grounded, and the first electric switch is turned off. After a power-on button is pressed down, a power supply on pin of the SIO chip sends a low level signal to turn off the second electric switch, the third electric switch is turned on, a power supply on pin of the connector is at a low level, and the computer is powered on. When the heat sink is installed improperly, the heat sink is not grounded, the first electric switch is turned on, the third electric switch is turned off, the power supply on pin of the connector is at a high level, and the computer cannot be powered on.