The invention relates to a gas bleed brake of an engine, which comprises a shell (01), a control valve (02), a check valve, a piston assembly (03) and an exhaust gate (04). The gas bleed brake is characterized in that the piston assembly comprises a piston sleeve (32), an implement piston (33), a return spring(34) and a piston blanking cover(36), wherein the piston sleeve (32) can freely move up and down in a piston cavity(14); the initial position of the piston sleeve is adjusted by a bolt part at the head part of the piston sleeve, and after adjustment, the piston sleeve is locked at the adjusted position by a locknut (10); the implement piston (33) can freely move up and down in the inner cavity of the piston sleeve and pressed at the bottom end of the piston sleeve by a compression spring(34); the moving interval of the implement piston is the gap H of the implement piston (33) and the piston blanking cover (36), and the implement piston (33) aligns with the exhaust gate (04). By controlling the exhaust gate to keep open, the invention reduces the work of gas in a combustion chamber to a piston to the maximum extent, lowers the rotating speed of the engine and the speed of an entire car, enhances the brake property and improves the traveling safety.