An apparatus for controlling a brake of a tractor towing a trailer includes a trailer sensor for sensing whether or not the trailer is mounted at the tractor, an auxiliary brake driver for driving an auxiliary brake of the tractor, a main brake driver for driving main brakes of the tractor and the trailer, and a controller for setting a ratio of braking force of the auxiliary brake to a reference value with respect to a required braking force from a smart cruise control (SCC) system and allocating the remaining required braking force to the main brakes of the tractor and the trailer in the case in which the trailer sensor senses that the trailer is mounted at the tractor.