A heterogeneous network integrating device and method thereof are provided. The device and method are applied in a communication environment overlapped by a mobile network covering range and a wireless network covering range and using the mobile network and wireless network to transmit downlink traffic. The heterogeneous network terminal enables the mobile network air interface and wireless network air interface at the same time when transmitting the downlink traffic and requests the mobile network end to transmit by the mobile network and wireless network at the same time. The mobile network end assigns the downlink traffic of the heterogeneous network integration terminal to the mobile network and the wireless network. This uses the bandwidth feature of mobile network and wireless network to promote transmission rate of downlink traffic and shorten user waiting time.