The invention discloses an air pressure engine, comprising an air compressor, an air cylinder, a convex plate crank mechanism, a main shaft and an engine body. The front part and the back part of the engine body are fixed respectively with a front end cover and a back end cover, the front end cover is connected fixedly with a sun gear, and two ends of the main shaft are arranged respectively on the front end cover and the back end cover, which are supported by a bearing. The main shaft is provided with a flywheel disc, on which a plurality of convex plate crank mechanisms are uniformly arranged; a convex plate in the mechanism is connected with the flywheel disc through a convex plate shaft; two ends of a large crank are connected respectively with the convex plate and one end of a small crank through a crank shaft; the other end of the small crank is connected with a planetary gear shaft; the planetary gear shaft is arranged on the flywheel disc through the bearing; and the planetary gear shaft is meshed with the sun gear. A plurality of corresponding cylinders are arranged on the engine body, and an end head of a piston rod is contacted with the convex plate. The air compressor is connected with the engine body, which supplies compressed air to the air cylinder through an air storage tank, the convex plate is pushed by the air cylinder to drive the large crank and the small crank to operate, and the flywheel disc is shook to rotate and drive the main shaft to rotate. The air pressure engine has the advantages of simple structure, energy economization and environmental protection, and becomes cleaning force instead of a combustion engine.