This application provides a data transmission method, a device, and a network system. The method is applied to a backbone device, and the backbone device is connected to at least two access devices. After obtaining first data that needs to be sent to a first user device, the backbone device determines a first tunnel interface identifier corresponding to the first user device. The first user device is a single-homing user device. The backbone device sends, based on the first tunnel interface identifier, a first data packet including the first data to a first access device of the at least two access devices. The first access device is configured with the first tunnel interface identifier. This can optimize a data forwarding path, implement traffic optimization for the single-homing user device, and reduce traffic pressure on the network system.