Abstract: For long endurance small unmanned aerial vehicle(UAV) engines that expend large amount of fuel and in cases of specified length of flight at invariable altitudes and velocity, a math model of piston engine fuel consumption is set up. Considering the coupling interaction among geometry, aerodynamic structure and weight, the initial gross weight of UAV is set for the optimization objective. Based on the multidisciplinary design optimization(MDO) platform ModelCenter, and use the genetic algorithm to carry on the parameters optimization design. The calculation result indicates that the system optimization model is reasonable and effective. It gives a reference to fuel calculation for small UAV of the long endurance and high fuel coefficient and to MDO.