The invention discloses a four-stroke rotor engine comprising an engine cylinder, an engine gas inlet, an engine igniter, an engine gas outlet, an engine rotor, an engine output shaft, a rotor gear ring, a center gear, a rotor seal end and a rotor seal strip. The engine gas inlet, the engine igniter and the engine gas outlet are installed above the engine cylinder. The engine output shaft is installed in the center of the engine rotor. The rotor gear ring is installed at the engine rotor. The center gear is fixed in the middle of the engine cylinder and meshes with the inside of the rotor gearring. The rotor seal end is arranged at the top end of the engine rotor. The rotor seal strip surrounds the engine rotor and is fixed to the rotor seal end. The tooth ratio of the rotor gear ring tothe center gear is changed into 2:1. The shapes of the engine cylinder and the engine rotor are changed, the movement track of the engine rotor and the shape of the inner wall of the engine cylinder are matched, the cylinder compression ratio of the engine is increased, and work efficiency of the engine is improved.