A method and device for determining an ACK/NACK feedback bit number, used when carriers having different time division duplex (TDD) uplink/downlink configurations are aggregated for a user equipment or the aggregated carriers includes at least one FDD carrier and at least one TDD carrier, and when the user equipment conducts ACK/NACK feedback on a physical uplink sharing channel (PUSCH) having a corresponding physical downlink control channel (PDCCH), the PDCCH containing a downlink assignment index (DAI) domain. The method comprises: determining whether the carriers grouped by the user equipment contain a first type carrier; if yes, then determining (I) of each carrier c according to a parameter indicating the number of ACK/NACK feedback subframes; if no, then acquiring (II) and the Mc of each carrier c, and selecting the minimum value from (II) and the Mc as the (I) of each carrier c; according to the number of the downlink subframes in each carrier c in need of conducting ACK/NACK feedback on the current uplink subframe, determining the ACK/NACK feedback bit number that the current uplink subframe needs to feed back. The present invention ensures normal operation of a system based on the application scenario.