A walking assistant device may calculate a walking cycle by measuring only motion of a specific joint without a force/torque sensor (F/T sensor) of a foot, and a method of controlling the walking assistant device. The method may include measuring motion of a hinge to which different support frames are connected; overlapping a reference trajectory corresponding to the measured motion and modulation trajectories that have been modulated from the reference trajectory; correcting the overlapping trajectory to correspond to the measured motion; determining assistant torque corresponding to a phase of the corrected trajectory; and providing the determined assistant torque to the support frame.