A link detection system for a network cable is applicable for detecting a connection state of the network cable. If the network cable is connected to the link detection system, the network state detection unit sends a voltage level of a first signal, so as to turn on the power supply of the LAN controller. If the network cable is unplugged, the LAN controller sends a second signal, so as to turn off the power supply of the LAN controller. Alternatively, if the network cable is linked to the network interface in an idle contact state, the network state detection unit sends another voltage level of the first signal, so as to not turn on the power supply of the LAN controller, thereby reducing the power consumption of the personal computer.