A data transmission method of a communication system, comprising: transmitting, by a first transmitting Radio Link Control, Tx RLC; entity of a first base station, a first Protocol Data Unit, PDU, to a first receiving RLC, Rx RLC, entity of a user equipment via a first leg; transmitting, by a second Tx RLC entity of a second base station, a second PDU to a second Rx RLC entity of the user equipment via a second leg, wherein the second PDU is a copy of the first PDU and the data transmission method of the communication system is further characterized by: respectively returning Acknowledgement, ACK, messages (ACK1, ACK2) from the first Rx RLC entity and the second Rx RLC entity to the first Tx RLC entity and the second Tx RLC entity, if the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU; wherein when the first Rx RLC entity does not successfully receive the first PDU within a predetermined time, the first Rx RLC entity transmits a first request message to the second Rx RLC entity, or when the second Rx RLC entity does not successfully receive the second PDU within the predetermined time, the second Rx RLC entity transmits a second request message to the first Rx RLC entity; wherein a request message is used for the first Rx RLC entity and the second Rx RLC entity to confirm whether the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU. transmitting, by a first transmitting Radio Link Control, Tx RLC; entity of a first base station, a first Protocol Data Unit, PDU, to a first receiving RLC, Rx RLC, entity of a user equipment via a first leg; transmitting, by a second Tx RLC entity of a second base station, a second PDU to a second Rx RLC entity of the user equipment via a second leg, wherein the second PDU is a copy of the first PDU and the data transmission method of the communication system is further characterized by: respectively returning Acknowledgement, ACK, messages (ACK1, ACK2) from the first Rx RLC entity and the second Rx RLC entity to the first Tx RLC entity and the second Tx RLC entity, if the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU; wherein when the first Rx RLC entity does not successfully receive the first PDU within a predetermined time, the first Rx RLC entity transmits a first request message to the second Rx RLC entity, or when the second Rx RLC entity does not successfully receive the second PDU within the predetermined time, the second Rx RLC entity transmits a second request message to the first Rx RLC entity; wherein a request message is used for the first Rx RLC entity and the second Rx RLC entity to confirm whether the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU. respectively returning Acknowledgement, ACK, messages (ACK1, ACK2) from the first Rx RLC entity and the second Rx RLC entity to the first Tx RLC entity and the second Tx RLC entity, if the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU; wherein when the first Rx RLC entity does not successfully receive the first PDU within a predetermined time, the first Rx RLC entity transmits a first request message to the second Rx RLC entity, or when the second Rx RLC entity does not successfully receive the second PDU within the predetermined time, the second Rx RLC entity transmits a second request message to the first Rx RLC entity; wherein a request message is used for the first Rx RLC entity and the second Rx RLC entity to confirm whether the first Rx RLC entity successfully receives the first PDU or the second Rx RLC entity successfully receives the second PDU.