The present invention relates to a wireless communication system, and more particularly to a method for enabling a station (STA) to perform a backoff in a wireless LAN system and an apparatus therefor. In a wireless LAN system according to an embodiment of the present invention, a method for enabling a station (STA) to perform a backoff includes the steps of: performing a first backoff procedure at a first primary channel for a transmission of a data unit having a size of a first channel width; performing a second backoff procedure at a second primary channel for a transmission of a data unit having a size equal to or greater than a second channel width; if a transmission opportunity (TXOP) is permitted as a result of the first backoff procedure, transmitting the data unit having a size of the first channel width; and if a TXOP is permitted as a result of the second backoff procedure, transmitting the data unit having a size equal to or greater than the second channel width.