Embodiments of the present application relate to the field of communications technologies, and provide a session management method and system, and a terminal. The method includes: when user equipment UE has a first protocol data unit PDU session, sending, by the UE, a service request message to an access and mobility management function AMF entity; receiving, by the UE, a first message sent by the AMF entity; and determining, by the UE based on the first message, to re-establish or modify the first PDU session. In the embodiments of the present invention, the UE sends the service request message to the AMF entity, and then the AMF entity sends, to the UE, the first message used to determine to re-establish or modify the first PDU session.