Abstract: In order to solve the irradiance uniformity problem of the solar simulator, a kind of square optical integrator was designed that could obviously improve the solar simulator irradiance uniformity. According to the composition and working principle of the solar simulator and optical integrator, the square optical integrator parameters were computed and the effect of integrator aberration on the uniformity of woring face was analyzed. Simulation of the solar simulator system was done using Lighttools software, and the optimization methods were proposed for improving irradiation uniformity by projection lens defocus.The irradiation nonuniformity can reach 1.72% in 200 mm200 mm square irradiation surface by using the designed integrator.