The present invention relates to a method and a device for beamforming, and, more particularly, to a method and a device for transmitting a downlink signal by using a two-dimensional active array antenna in a wireless communication system. In order to achieve the objective mentioned above, a beamforming method by a base station in a mobile communication system, according to one embodiment of the present invention, comprises the steps of: receiving an uplink signal from at least one terminal; determining directivity information of a vertical channel of a downlink for the at least one terminal on the basis of the uplink signal; and performing resource scheduling and beamforming for the at least one terminal on the basis of the directivity information of the vertical channel of the downlink and channel status information in a horizontal direction received from the at least one terminal. According to an embodiment of the present invention, since a base station can estimate vertical channel information by using an uplink signal, it is possible to provide three-dimensional beamforming to terminals even without receiving the vertical channel information directly from the terminals, and thus the signal quality of each user can be improved and the efficiency of spatial multiplexing in a cell can be increased to thereby increase the total network capacity.