A data transmission method, comprising: Sending (201, 504), by first user equipment UE, a first message to a core network entity, for the core network entity obtaining a bearer context of the first UE and setting up a target bearer for the first UE between the first UE and a serving gateway, wherein the first message is used by the first UE to request to change from accessing a network by using second UE to directly accessing the network; and Transmitting, by the first UE, data by using the target bearer; wherein before the sending, by first user equipment UE, a first message to a core network entity, the method further comprises: sending, by the first UE, a seventh message to the second UE, wherein the seventh message is used to request the second UE to send network information to the first UE; receiving, by the first UE, the network information sent by the second UE; accessing, by the first UE, the network by using the network information; setting up, by the first UE, a radio bearer between the first UE and a base station; and receiving, by the first UE, a TAU confirm message sent from the network entity for setting up the target bearer on the first UE. wherein the network information comprises a globally unique temporary UE identity GUTI of the first UE; and wherein the first message is a tracking area update TAU message and includes the GUTI. Sending (201, 504), by first user equipment UE, a first message to a core network entity, for the core network entity obtaining a bearer context of the first UE and setting up a target bearer for the first UE between the first UE and a serving gateway, wherein the first message is used by the first UE to request to change from accessing a network by using second UE to directly accessing the network; and Transmitting, by the first UE, data by using the target bearer; wherein before the sending, by first user equipment UE, a first message to a core network entity, the method further comprises: sending, by the first UE, a seventh message to the second UE, wherein the seventh message is used to request the second UE to send network information to the first UE; receiving, by the first UE, the network information sent by the second UE; accessing, by the first UE, the network by using the network information; setting up, by the first UE, a radio bearer between the first UE and a base station; and receiving, by the first UE, a TAU confirm message sent from the network entity for setting up the target bearer on the first UE. wherein the network information comprises a globally unique temporary UE identity GUTI of the first UE; and wherein the first message is a tracking area update TAU message and includes the GUTI. sending, by the first UE, a seventh message to the second UE, wherein the seventh message is used to request the second UE to send network information to the first UE; receiving, by the first UE, the network information sent by the second UE; accessing, by the first UE, the network by using the network information; setting up, by the first UE, a radio bearer between the first UE and a base station; and receiving, by the first UE, a TAU confirm message sent from the network entity for setting up the target bearer on the first UE. wherein the network information comprises a globally unique temporary UE identity GUTI of the first UE; and wherein the first message is a tracking area update TAU message and includes the GUTI.