Provided is a method and apparatus for transmitting an HARQ ACK/NACK. The method includes: in a TDD-FDD CA scheme; recognizing a 2-bit downlink (DL) downlink assignment index (DAI) field configured in a DL downlink control information (DCI) format, the DL DCI format indicating a Physical Downlink Shared Channel (PDSCH) transmission on the second serving cell, and the 2-bit DL DAI field indicating that ten downlink subframes for the second serving cell are associated with one uplink subframe; in response to received data, generating a Hybrid Automatic Repeat reQuest (HARQ) Acknowledgement/Negative Acknowledgement (ACK/NACK) signal, the HARQ ACK/NACK signal being indexed based on a value of the 2-bit DL DAI field; and transmitting the HARQ ACK/NACK signal through one uplink subframe of the first serving cell.