The topography of plateau regions is complex. In order to avoid obstacles better, monitoring the altitude of UAV real-time is very important. This paper puts forward a high accuracy and fault-tolerance altitude measuring system based on information fusion. Three kinds of sensors: GPS, barometric altimeter and IMU are used to measure the altitude of plateau UAV based on the method of redundancy and information fusion. The principle and model of the three sensors are analyzed and the method of altitude information fusion is studied based on the federal filter. On this basis, the information prediction and fusion process of the three kinds of sensor is further discussed and the altitude measuring system is simulated. The results of simulation show that the design of the altitude measuring system is reasonable and satisfied with the requirements of flight quality, with further engineering applications.