A method, a device, and a system for controlling quality of service are provided. The method includes: obtaining, by a policy and charging enforcement function PCEF entity, a service flow priority identifier (FPI) of a downlink data packet, where the FPI is used to indicate a priority of a service flow to which the downlink data packet belongs; carrying, by the PCEF, the FPI in the downlink data packet; and sending, by the PCEF, the downlink data packet to a base station, so that the base station sends the downlink data packet to a user equipment UE according to the FPI. By adopting the method, the device, and the system according to the embodiments of the present invention, end-to-end differentiated processing of a service flow can be implemented, differentiated experiences can be provided for different levels of subscribers and different types of services.