A congestion notification method, a related device, and a system, where the method includes receiving, by a network side device, a congestion status of a target network area sent by a radio access network (RAN) congestion awareness function (RCAF) entity, where the target network area is a network area involved in an application service provided by a target application server; and sending, by the network side device, the congestion status of the target network area to the target application server. Hence, an application server may able to learn in time when congestion occurs, and adjust, according to the congestion situation, a related service in an area corresponding to the congestion situation.