A method for uplink multi-user (UL MU) operation in a WLAN system according to an embodiment of the present specification comprises the steps of: receiving, by an access point (AP), a buffer state report frame used for the UL MU operation from a user STA associated with the AP, wherein the buffer state report frame includes bitmap information for a plurality of access categories for which the buffer state of the user STA is to be reported, and identification information for a number of traffic identifiers (TIDs) for at least one traffic being buffered by the user STA, wherein the identifier information is set based on a number of the plurality of access categories indicated by the bitmap information; and transmitting, by the AP, a trigger frame for triggering the UL MU operation from a plurality of user STAs.