A packet transmission method, device, and system are disclosed, and pertain to the field of network technologies. The system includes a first network device in a VPLS network and a second network device in a VPWS network. The first network device receives, by using a first VPLS instance in the first network device, a packet to be sent to a user-side device connected to the second network device, determines a first VPWS instance in the first network device based on an association relationship between the first VPLS instance and the first VPWS instance, and forwards the packet to a second VPWS instance in the second network device by using the first VPWS instance. A VPLS instance is associated with a VPWS instance in the first network device, so that a packet that is received by using the VPLS instance and that is to be sent to a user-side device can be forwarded by using the associated VPWS instance, pressure on the first network device to learn a MAC address for packet transmission is reduced, and forwarding performance of the system is ensured.