Optical integrator is a key optical device of the solar simulator for uniform illumination. The composition and working principle of the optical integrator are introduced, and effect of the azimuth offset error and the tilt offset error of on the irradiation uniformity are elaborated. According to the working principle of the optical integrator, effect of two axis-consistency errors on the irradiation uniformity are analysed by using the LightTools. Combining with experimental results, effect of tilt offset error relative azimuth error on the irradiation uniformity of system is more serious. Analysis of axis consistency error can provide the necessary reference for tuning mechanism of the optical integrator and the practical alignment.