The present invention discloses a method and device for downlink multi-user (DL MU) transmission in a wireless communication system. In particular, the method for downlink multi-user data transmission in the wireless communication system includes the steps of receiving, by a station (STA), a DL MU data frame from an access point (AP) and transmitting, by the STA, an acknowledgement (ACK) frame in response to the DL MU data frame, wherein a plurality of ACK frames transmitted by a plurality of STAs in response to the DL MU data frame is multiplexed to configure an uplink multi-user (UP MU) ACK frame, and the time and/or size of a frequency resource at which the ACK frame is transmitted may be determined according to the level of a modulation and coding scheme (MCS) that is applied to the ACK frame.