Embodiments of the present invention provide a data transmission method, device, and system. A first device is configured to serve a primary carrier, and the first device includes: a first unit configured to process an RLC layer, a second unit configured to process a MAC layer, a third unit configured to process a PDCP layer, and a first scheduling unit. The first unit includes a first control module and a first transition module. The second transition module is configured to: receive the N data packets and scheduling information sent by all scheduling units in the system, acquire, according to all or a part of the received scheduling information and from the N data packets a data packet that needs to be transmitted on the secondary carrier, and send the data packet acquired by the second transition module to the fifth unit.