Disclosed are an uplink data transmission method in a wireless communication system and a device therefor. Particularly, a method for transmitting uplink data in an unlicensed band by a user equipment (UE) in a wireless communication system may comprise the steps of: receiving, from a base station, uplink scheduling information within a cell operating in an unlicensed band; sensing a channel of the cell; transmitting a reservation signal when the channel is sensed to be in an idle state; and transmitting, to the base station, uplink data in a wireless resource allocated on the basis of the uplink scheduling information, wherein the channel is sensed to be in the idle state when energy detected, except for the reservation signal transmitted from another UE that is co-scheduled along with the UE for uplink multi-user multiple input multiple output (UL MU-MIMO) transmission, is smaller than a predetermined threshold value.