A system for maintaining a constant temperature of an engine in an unmanned aerial system with an autothrottle limiting device comprises an autopilot for issuing a throttle command, an autothrottle limiting device for automatically limiting an upper limit of the throttle command issued by the autopilot, and a rotary engine for feeding an internal temperature of the engine back to the autothrottle limiting device. The system is applicable to all kinds of unmanned aerial systems employing an air-cooling rotary engine, uses the existing autopilot of the unmanned aerial system without modification, and improves the reliability and life of the engine.