A method for data transmission, comprising: receiving, by a first primary network device, measurement result information of a secondary network device transmitted by a terminal device in a process that the terminal device accesses the first primary network device, the measurement result information being configured to indicate link quality between the secondary network device and the terminal device; determining, by the first primary network device, to establish a secondary connection between the secondary network device and the terminal device according to the link quality between the secondary network device and the terminal device; and establishing, by the first primary network device, the secondary connection: wherein establishing, by the first primary network device, the secondary connection comprises: transmitting, by the first primary network device, a secondary connection addition request message to the secondary network device, the secondary connection addition request message being configured to request the secondary network device to be added as a network device for data transmission; receiving, by the first primary network device, an acknowledgement message of the secondary connection addition request message from the secondary network device, wherein the acknowledgement message of the secondary connection addition request message carries configuration information of the secondary connection, the configuration information of the secondary connection being configured to establish the secondary connection between the secondary network device and the terminal device; acquiring, by the first primary network device, the configuration information of the secondary connection; and transmitting, by the first primary network device, the configuration information of the secondary connection to the terminal device, wherein the secondary connection is used for offloading control-plane data for the first primary network device, or control plane data and user-plane data for the first primary network device. receiving, by a first primary network device, measurement result information of a secondary network device transmitted by a terminal device in a process that the terminal device accesses the first primary network device, the measurement result information being configured to indicate link quality between the secondary network device and the terminal device; determining, by the first primary network device, to establish a secondary connection between the secondary network device and the terminal device according to the link quality between the secondary network device and the terminal device; and establishing, by the first primary network device, the secondary connection: wherein establishing, by the first primary network device, the secondary connection comprises: transmitting, by the first primary network device, a secondary connection addition request message to the secondary network device, the secondary connection addition request message being configured to request the secondary network device to be added as a network device for data transmission; receiving, by the first primary network device, an acknowledgement message of the secondary connection addition request message from the secondary network device, wherein the acknowledgement message of the secondary connection addition request message carries configuration information of the secondary connection, the configuration information of the secondary connection being configured to establish the secondary connection between the secondary network device and the terminal device; acquiring, by the first primary network device, the configuration information of the secondary connection; and transmitting, by the first primary network device, the configuration information of the secondary connection to the terminal device, wherein the secondary connection is used for offloading control-plane data for the first primary network device, or control plane data and user-plane data for the first primary network device. transmitting, by the first primary network device, a secondary connection addition request message to the secondary network device, the secondary connection addition request message being configured to request the secondary network device to be added as a network device for data transmission; receiving, by the first primary network device, an acknowledgement message of the secondary connection addition request message from the secondary network device, wherein the acknowledgement message of the secondary connection addition request message carries configuration information of the secondary connection, the configuration information of the secondary connection being configured to establish the secondary connection between the secondary network device and the terminal device; acquiring, by the first primary network device, the configuration information of the secondary connection; and transmitting, by the first primary network device, the configuration information of the secondary connection to the terminal device, wherein the secondary connection is used for offloading control-plane data for the first primary network device, or control plane data and user-plane data for the first primary network device.