A braking device for a rotating shaft and a gear motor system using the same. The braking device includes a brake subassembly, a rotating shaft, and a sleeve. The rotating shaft is sheathed with the sleeve. The brake subassembly includes an adjustable screw, a spring, and a brake disc. One end of the spring presses on top of the brake disc and the other end presses at the bottom of the adjustable screw. After the adjustable screw is fixed, the bottom surface of the brake disc is tightly against the outer surface of the sleeve. When the adjustable screw is adjusted, the amount of compression of the spring is changed and the pressure imposed on the brake disc by the spring varies accordingly such that the friction between the brake disc and the sleeve is changed.