The present invention relates to a wireless communication system. Specifically, the present invention relates to a method for transmitting, by a terminal, a HARQ-ACK in a CA wireless communication system and an apparatus therefor, the method comprising: receiving at SF #n-k one or more DG DCIS containing a first DAI; receiving a UG DCI which schedules a PUSCH; and transmitting at SF #n a HARQ-ACK payload over the PUSCH, wherein the size of the HARQ-ACK payload is determined on the basis of a value of the first DAI, the value of the first DAI corresponds to the total number of cells on which a first type of DL signal is scheduled at the SF #n-k with respect to the terminal, n is an integer of 0 or more and k is a positive integer.