In order to make the medium-altitude, long-endurance (MALE)UAV have good dynamic and economic performance in all flight stages, combining the speed characteristic curve, fuel consumption characteristic curve of piston engine and propulsion characteristic curve of propeller, the method of matching constant speed propeller for aero-piston engine is proposed. According to the speed characteristic curve fuel consumption characteristic curve of piston engine, the economic working points of the engine are selected and then efficiencies of each propeller on the economic working points above are compared and analyzed. Wind tunnel tests of full-scale engine and propeller showed that the proposed method could be used to select optimal constant propeller for piston engine which had been preliminarily selected.