Disclosed are a method and device for performing a sensing procedure in a wireless LAN system. A method for a first station (STA) to perform a sensing procedure in a wireless LAN system comprises the steps of: receiving, from a second STA, first information related to the number of LTF symbols repeated for each spatial stream; and receiving at least one NDP based on the first information from the second STA, wherein the total number of the LTF symbols included in each of the at least one NDP may be based on the number of the spatial streams and the first information.