A data transmission system, wherein the system comprises a first device and a second device, wherein: the first device is configured to serve a primary carrier of user equipment, and the first device comprises: a first unit that is configured to process a radio link control RLC layer, a second unit that is configured to process a Media Access Control MAC layer, a third unit that is configured to process a Packet Data Convergence Protocol PDCP layer, and a first scheduling unit, wherein the first unit comprises a first control module and a first transition module; the first scheduling unit is configured to send first scheduling information to all transition modules in the system, and the first scheduling information comprises scheduling information of the primary carrier; the first control module is configured to: receive data from the third unit, group the data into N data packets, and send the N data packets to all the transition modules in the system; the first 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 primary carrier, and send the data packet acquired by the first transition module to the second unit; the second device is configured to serve a secondary carrier of the user equipment, and the second device comprises a fourth unit that is configured to process the RLC layer, a fifth unit that is configured to process the MAC layer, and a second scheduling unit, wherein the fourth unit comprises a second transition module; the second scheduling unit is configured to send second scheduling information to all the transition modules in the system, and the second scheduling information comprises scheduling information of the secondary carrier; and the second transition module is configured to: receive the N data packets and the scheduling information sent by all the 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. the first device is configured to serve a primary carrier of user equipment, and the first device comprises: a first unit that is configured to process a radio link control RLC layer, a second unit that is configured to process a Media Access Control MAC layer, a third unit that is configured to process a Packet Data Convergence Protocol PDCP layer, and a first scheduling unit, wherein the first unit comprises a first control module and a first transition module; the first scheduling unit is configured to send first scheduling information to all transition modules in the system, and the first scheduling information comprises scheduling information of the primary carrier; the first control module is configured to: receive data from the third unit, group the data into N data packets, and send the N data packets to all the transition modules in the system; the first 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 primary carrier, and send the data packet acquired by the first transition module to the second unit; the second device is configured to serve a secondary carrier of the user equipment, and the second device comprises a fourth unit that is configured to process the RLC layer, a fifth unit that is configured to process the MAC layer, and a second scheduling unit, wherein the fourth unit comprises a second transition module; the second scheduling unit is configured to send second scheduling information to all the transition modules in the system, and the second scheduling information comprises scheduling information of the secondary carrier; and the second transition module is configured to: receive the N data packets and the scheduling information sent by all the 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.