Embodiments of the present invention provide a method, network element device, and network system for associating a terminal device with a network. The method for associating a terminal device with a network includes: receiving an Association Request message sent by a terminal device, where the Association Request message includes information of a request for associating the terminal device with a sensor network and an IP identifier; processing, according to the Association Request message, the request for associating the terminal device with a sensor network; when it is determined, according to the IP identifier, that the terminal device requests association with an IP network, sending an IP network Association Request message to an Edge Router; receiving a result of processing, by the Edge Router, the request for associating the terminal device with the IP network; sending an Association Respond message to the terminal device.