Aimed at the problems that the accuracy of energy acquisition process is low and the design parameters are difficult to be optimized in the overall design of small longendurance solar powered unmanned aerial vehicle (UAV), an optimization method of the overall design in small lowaltitude longendurance solar powered UAV is put forward based on the principle of energy balance. In order to accurately characterize the solar radiation intensity, the atmospheric mass and atmospheric transparency coefficient are introduced. A solar radiation model of the UAV is established with direct radiation intensity and scattered radiation intensity. Based on the principle of solar energy, combining with solar radiation model and energy relationship, statistical analysis of UAV mass distribution and parametric modeling, the energy balance model is established and verified. A concept of remaining time after crossnight flight is proposed, and the optimal design parameters and schemes are obtained by using the genetic algorithm to optimize the maximum remaining time after crossnight flight. Taking the flying in Xi’an in June 22nd as an example, the optimal design is carried out, and the prototype is developed to carry out flight test. The result shows that the prototype has the ability of crossnight flight.