Embodiments of the present invention provide a method for processing a rate group, a method for charging for a data service, and a related device and system, so as to enable a charging system to accurately charge for the data service when the rate group changes. The method includes: determining, by a gateway device, whether a rate group of a service data stream changes; and if the rate group of the service data stream changes, reporting, by the gateway device, charging data to the charging system, so that the charging system performs charging according to the charging data, where the charging data includes service usage information and a reporting reason for reporting the charging data, and the reporting reason indicates a change of the rate group.