A data forwarding method and system is provided for avoiding data loss during a vertical handover by forwarding data through an IP tunnel established between a newly introduced network entity called a Data Forwarding Function of the serving network and a Forward Attachment Function of a target network. A data forwarding method of a data forwarding unit located in a serving network for a vertical handover according to the present invention includes receiving an address of a forwarding attachment unit located in a target network and a data forwarding preparation request that are transmitted by a user equipment; establishing an Internet Protocol (IP) tunnel to the forwarding attachment unit according to the data forwarding preparation request; and forwarding, when a vertical handover is initiated, handover packets to the forwarding attachment unit through the IP tunnel.