Proposed are a method and a device for transmitting a signal in a wireless local area network (WLAN) system. Specifically, a first STA transmits at least one BRP frame in order to perform MIMO beamforming training with a second STA. The at least one BRP frame is transmitted during a TDD-based SP. The first STA transmits a signal to the second STA based on MIMO beamforming training. The SP includes a first TDD slot and a second TDD slot. If only a first BRP frame, which is part of the at least one BRP frame, can be transmitted in the first TDD slot, a second BRP frame, which is the remaining of the at least one BRP frame, is transmitted in the second TDD. The second TDD slot is a transmission TDD slot allocated first after the first TDD slot.