Compared to conventional powered aircraft, the solar-powered aircraft have characteristics of high-altitude and long-endurance. They can modularly change the task loads and carry out related special tasks. Based on the working principle of long endurance solar-powered aircraft, the coupling relationship between the all-day cruising altitude and wing load of solar-powered aircraft under the energy balance design system was analyzed. The coupling parameters, including aerodynamic parameters, solar panels efficiency and paving rate, propulsion system efficiency and load power factor, flight season and flight latitude, were systematically analyzed for sensitivity, The results indicate that the design or optimization of appropriate lift and drag coefficients should be the first consideration for long endurance solar-powered aircraft to achieve best aerodynamic efficiency; When the efficiency of solar panels reaches 0.35 or above, increasing the installation rate of solar panels has a weaker impact on the all-day cruising altitude, but it helps to increase the upper limit of wing load. The research results can serve as a reference for the overall design and improvement optimization of solar-powered aircraft.