Disclosed are a data transmission method and device in a carrier aggregation system, used for realizing the data transmission of UE which does not support uplink and downlink transmission simultaneously in a carrier aggregation system which uses different TDD uplink and downlink configurations. The data transmission method in a carrier aggregation system provided in the present application includes: an eNB performing data transmission with specific user equipment (UE) according to the time division duplex (TDD) uplink and downlink sub-frame configuration employed by a specific component carrier in a carrier aggregation system by means of a component carrier in the carrier aggregation system, wherein said specific UE is TDDUE which does not support simultaneously performing uplink data transmission and downlink data transmission in an identical sub-frame.