The invention discloses a system and a method for controlling fixed-height flight of a fixed-wing unmanned aerial vehicle, relating to the field of unmanned aerial vehicle control. By using the system and the method, a barometric height sensor is used as a height measuring element, an external lithium battery is used for supplying power to the whole system, and the sensor is protected by using a wind filtering device to reduce the influence, caused by wind pressure at a high altitude, on the sensor so that errors are reduced. After related information is measured by the barometric height sensor, an I2C bus (internal integrated circuit bus) transmits data to a single-chip microcomputer system, a series of data calculations are performed in a single-chip microcomputer to obtain corresponding height information; and finally, an elevating rudder and a gasoline throttle on the plane are linked together for PD (proportional plus derivative) control so as to control angle changes of a steering engine and the size of the gasoline throttle, and a height over-ascending and over-descending protection program is added in the programs to further improve the safety and realize autonomous height fixing of the unmanned aerial vehicle. The system has the advantages of low cost, small error, stable structure, small interference from external environment, easy implementation and the like.