A method, a device, and a system for implementing a Mux machine are provided. The method includes: setting, by a first network, a Mux machine on a first aggregation port to be in a PROTECT_WAITING state; when the Mux machine is in the PROTECT _WAITING state, determining, by the first network device, that the first aggregation port is not in a collecting and distributing state, and sending, to a second network device, a first LACPDU packet that is used to notify that the first aggregation port is in the collecting and distributing state; and when determining that a second LACPDU packet that is from the second network device and that is used to indicate that a second aggregation port is in the collecting and distributing state is received, switching, by the first network device, a status of the Mux machine from the PROTECT WAITING state to a COLLECTING DISTRIBUTING state, and setting the first aggregation port to be in the collecting and distributing state. In implementations of this application, Single-port UP of an aggregated link in the Mux machine is detected and processed, thereby alleviating a packet loss generated during service traffic transmission.