The disclosure relates to data transmission method and apparatus for transmitting UL grant or ACK/NACK at flexible subframes in a Time Division Duplex (TDD) system. The data transmission method of a base station according to the present invention includes configuring subframes irrelevant to Hybrid Automatic Repeat reQuest (HARQ) process as flexible subframes; scheduling uplink transmission at the flexible subframes with uplink grant; receiving uplink data transmitted by a terminal at the flexible subframes as scheduled; and transmitting, when decoding the uplink data fails, HARQ acknowledgement/negative acknowledgement (ACK/NACK) or uplink grant to request retransmission of the uplink data. The data transmission method of the present invention is capable of utilizing flexible subframes for uplink transmission without influencing the uplink HARQ process associated with the downlink subframes configured as flexible subframes.