Embodiments of the present invention disclose a service data transmission method. An AN entity of a first network sends a first request message to an AN entity of a second network corresponding to a target cell in which a UE is located, so as to request the AN entity of the second network to establish a radio bearer for the UE. The AN entity of the first network sends a second request message to the UE, so as to request the UE to establish the radio bearer with the AN entity of the second network. After the AN entity of the second network has established the radio bearer with the UE, the AN entity of the first network transmits service data to the UE by using the AN entity of the second network.