The invention discloses an engine, an in-cylinder brake system and an air valve bridge assembly. The air valve bridge assembly comprises a main piston mounting hole and a brake piston mounting hole, wherein the main piston mounting hole is formed in the inner side end of an air valve bridge body; the brake piston mounting hole is formed below the main piston mounting hole and communicates with themain piston mounting hole; an oil unloading assembly mounting hole is formed in the outer side end; an air valve bridge oil duct communicating the piston mounting hole with the oil unloading assemblymounting hole is formed in the air valve bridge body; a main piston assembly is mounted in the main piston mounting hole; a brake piston assembly is mounted in the brake piston mounting hole; an oilunloading assembly is mounted in the oil unloading assembly mounting hole; the in-cylinder brake system comprises an exhausting rocker arm assembly and an air valve bridge assembly; and a ball pin assembly keeping contact with the main piston assembly and an adjusting screw assembly keeping contact with the oil unloading assembly are arranged on the air valve side of the rocker arm body of the exhausting rocker arm assembly. The load transmitted to a push rod side by the in-cylinder brake system is greatly reduced, so that separation of the rocker arm and the air valve bridge is prevented, thefailure that a rocker arm elephant foot overturns and is knocked to be damaged is avoided, and the reliability is greatly improved.