A method and a device for monitoring a service usage amount includes: determining, according to one of quintuple information, protocol information, and URL information in received service request information, whether the service request information is the same as specified service stream information in one or multiple session level service monitoring tasks, where the session level service monitoring task includes a session level service monitoring class identity, a monitoring task identity, and the specified service stream information; the monitoring class identity indicates that the monitoring task is used to monitor a usage amount of a specified service stream in a session; when determining yes, accumulating a usage amount of the service request information in the one or multiple session level service monitoring tasks; and reporting the monitoring task identity and an accumulated usage amount of the service request information in the session level service monitoring task to a PCRF.