A network-based mobility system and a handover method in the system are provided. If a mobile terminal having interfaces of different service providers establishes a connection between domains of the different service providers, a new domain to which the mobile terminal is connected sends a notification of the new connection to the existing domain to which the mobile terminal has been connected. If there is a flow binding switch operation caused by the new connection, the existing domain sends a flow binding setup request to the new domain. The new domain sets up flow binding for the mobile terminal. A tunnel for the corresponding flow is set up between the new domain and the existing domain, whereby packets transmitted to the existing domain are forwarded to the new domain through the tunnel.