In order to solve the problem of low radiation energy at the edge of the output spot of the equal curvature optical integrator, which leads to the poor uniformity of the radiation surface, the differential and integration method of radiation energy based on the diffraction theory is proposed, and a variable curvature optical integrator is designed. The aperture and number of the integrator channel are determined according to the Fresnel number, and the radiation distribution curve is divided equidistant. According to the difference of the focal length of each eye lens in the integrator, the radiation distribution curve is superimposed step by step. Based on the diffraction theory, the mathematical model of variable curvature optical integrator in two-dimensional plane is established, and the mathematical function of light intensity distribution on the working face is deduced. The aspheric optimization design of each circle eye lens in the field lens group is carried out by using Zemax software to improve the imaging quality and eliminate the sidelobe effect. The variable curvature optical integrator and the constant curvature optical integrator are simulated by LightTools software, and their performance differences are compared and analyzed. The results show that the variable curvature optical integrator can significantly improve the edge radiation energy of the output spot of the solar simulator, up to 56% higher than that of the equal curvature optical integrator, the irradiation non-uniformity in the Φ100 mm radiation plane is better than ±0.5%, and the radiation inhomogeneity in the Φ200 mm radiation plane is better than ±1%.