Embodiments of this application provide a packet forwarding method, a packet processing method, and a device, to determine a network slice that corresponds to a packet and that is of a transport network. The packet forwarding method includes the following steps: A first device obtains a first packet. The first device is a network device in an access network or in a mobile core network. The first device determines network slice mapping information based on the first packet. The first device generates a second packet based on the first packet. The second packet includes the network slice mapping information. The first device sends the second packet to a second device. The second device is a network device in a transport network.