The invention relates to a transmission gap adjusting mechanism of a gantry numerical control cutting machine, which is mainly as follows: bearings are mounted in a mounting seat, a spacing sleeve is used for positioning between the bearings, the spacing sleeve compresses outer rings of the bearings, a speed reducer is fixed on the mounting seat, a gear is connected with an output shaft of the speed reducer, a fixed seat is arranged in a copper sleeve, the mounting seat is connected with the fixed seat by a rotating shaft, the fixed seat is mounted on a frame by a first bolt and a second bolt, a screw rod is screwed in the frame, a spring is arranged in the screw rod, a nut is screwed in and compresses the gear, and the gear is meshed with a rack. The first bolt is used for regulating the verticality between the gear and the rack. The transmission gap adjusting mechanism of the numerical control cutting machine has simple structure, convenient regulation and large assembly space, and can facilitate the processing of the frame, improve the rigidity of the frame, reduce the phenomenon of fracture of the expensive output shaft of a reduction gearbox, reduce the maintenance ratio of replacing the reduction gearbox, further reduce the manufacturing cost and improve the product quality.