A data transmission method and a data transmission device for a TDD system are provided, so as to reduce the frequency hopping operations, reduce the retuning time, prolong a valid duration for the data transmission and reduce the time period for the data transmission, and reduce the power consumption. The data transmission method includes steps of: at a predetermined retuning start time point, adjusting, by a UE, a carrier frequency for data reception and transmission within a predetermined retuning duration so as to acquire the adjusted carrier frequency; and performing, by the UE, uplink data transmission and downlink data reception at the adjusted carrier frequency with a network device within a consecutive duration in accordance with a TDD uplink-downlink configuration. The consecutive duration is a duration from a current retuning end time point to a next retuning start time point.