A method for preventing unauthorized connection in a network system mainly includes adding an authentication key in the LLDP (link layer discovery protocol) transmitted in accordance with the 802.1ab communication protocol so as to proceed with security mechanism under the structure of 802.1ab communication protocol. The method for preventing unauthorized connection includes receiving a LLDP packet satisfying the 802.1ab communication protocol transmitted from a second network device by a first network device in a network system; analyzing the LLDP packet and checking whether the LLDP packet contains a legitimate authentication key; and if the authentication key does not exist or is illegitimate, then block all packets transmitted from the second network device so as to prevent the unauthorized second network device from using the network transmission service provided by the first network device.