A data push service system and method using a heterogeneous network. The system transmits push data to a Hybrid Access Terminal (HAT) in a heterogeneous mobile communication system in which at least one circuit network overlaps with at least one packet network. The HAT can be connected to a circuit network or a packet network, and is registered in a new access network. The system includes a Packet Service & Mobility Gateway (PSMG) for acquiring access state and connection information of the HAT from the circuit network and the packet network, and generating an access network switching request upon receipt of an external request signal; and a Push Proxy Gateway (PPG) for acquiring access information of the HAT which will transmit the push data from the PSMG, and transmitting a request message to the PSMG.