Embodiments of the present disclosure provide a session activation method, a mobility management function entity, a session management function entity, a terminal, and a system. The method includes: obtaining, by a mobility management function entity, a protocol data unit (PDU) session activation request of a terminal, where the PDU session activation request includes an identifier of a PDU session and an identifier of the terminal, and the PDU session activation request is used to request activation of the PDU session; sending, by the mobility management function entity, the PDU session activation request to a session management function entity; and activating, by the session management function entity, the PDU session based on the PDU session activation request.