The embodiment of the invention discloses an automatic online method of an SDN device, the SDN device and a controller, and relates to the field of communication technology. Automatic online of the SDN device is realized to simplify the device online working flow in an SDN, improve the device online efficiency and realize plug and play and plug and management of the device. The method comprises the following steps: broadcasting a DHCP discovery message packaged with a virtual local area network VLAN identifier by an to-be-online SDN device; receiving a first DHCP response message broadcasted by a first controller; setting a first port receiving the first DHCP response message in the VLAN identifier; broadcasting a DHCP request message by using the first port; receiving a DHCP answer message broadcasted by the first controller through the first port; and after the SDN device establishes an OpenFlow connection with the controller, receiving an outbound message sent by the controller, wherein the outbound message carries a link layer discovery protocol LLDP message.