The invention provides an air circulating rotor engine. The engine is designed by a new method of combining air energy and mechanical energy, in the development process, only when the supply and the demand of the energy are inconsistent, compressed air can be circulated, and the aim that the engine does work is achieved. The air circulating rotor engine is characterized in that mechanical energy is used for converting air into air energy on the same engine body, and therefore different work-doing effects can be achieved through five differences and one conversion. The five differences comprise different kinds of work doing of the same engine body, different axes of the same engine body, different speeds of the same rotating, different pressures of the same air and different volumes of the same cylinder. According to one conversion, the mechanical energy converts the air into the air energy, and therefore the air pushes a piston connecting rod eccentric wheel or shaft of the engine to do the reciprocating motion so as to drive a gear to rotate and circularly do work. According to the operation work-doing principle, an electric compressor is used for enabling an air storage bottle to be filled with the air in a pressing mode, the air storage bottle is used as a starting air source to push the engine to do work and drive the compressor to make the air, the air made by the compressor is pressurized by a booster pump and then enters the air storage bottle, the air is controlled by a control valve to be supplied to an engine piston to do work, and therefore the aim that the air of the engine circularly does work is achieved.