Embodiments of this application provide a method, an apparatus, and a system for reporting data usage, so that for a PDU session into which a UL CL user plane network element or a BP user plane network element is inserted, statistics can be accurately collected on data usage. The method includes: An aggregate user plane network element determines discarded data usage of the aggregate user plane network element, and sends the discarded data usage to a session management network element. The first session management network element receives the discarded data usage, and receives transport data usage of one or a plurality of anchor user plane network elements, which are anchor user plane network elements connected to the aggregate user plane network element. The first session management network element determines actual data usage of a terminal device based on the discarded data usage and the transport data usage.