A method for downlink beamforming in a wireless access system and a device therefor are disclosed. Specifically, the method comprises the following steps: transmitting, by an eNB, a reference signal to a UE through only a first antenna set from among whole antennas; receiving, by the eNB, channel state information (CSI) for the first antenna set from the UE; and transmitting, by the eNB, beamforming a downlink channel by using a precoding matrix for the whole antennas, wherein CSI for a second antenna set by which the reference signal has not been transmitted from among the whole antennas is calculated using the CSI for the first antenna set.