An optical system simulation and design methods for large-area divergent solar simulators were studied to achieve uniform illumination of a large irradiation area of solar simulators. First, the working principles of fly-eye lens array groups and examine divergent projection systems as well as the production mechanism of the sidelobe effect was analyzed. Subsequently, using nesting modeling combined with the polynomial fitting method, the intensity distribution curve of the xenon lamp axis was obtained. Then a luminous intensity distribution simulation of the xenon lamp was conduct based on the luminous energy symmetry of the xenon lamp. Considering the boundary conditions and parameters for the optical system designly, a beam-shaping system, a fly-eye lens array group, and a divergent projection system were designed. The experimental results show that when the working distance is 20 000 mm and the irradiation surface diameter is 1 500 mm, the irradiation uniformity is 92.8%, which meets the uniform illumination requirement for large-area divergent solar simulators.