An optical system of LED solar simulator for photovoltaic performance test is designed by using LED source. A method of employing LEDs with different peak wavelengths as light source to build a solar spectrum is introduced. By taking use of the characteristics of the LEDs, a concentric transmission collimating system is used, including high power LEDs, collimating system, Fresnel lens, optical integrator and aperture. The free-form surface collimating system with the total internal reflection of free-form surface lens and symmetrical lens array structure in optical integrator is adopted. The results demonstrated by optical simulation software of TracePro show that in the effective radiation circle area within 220 mm diameter, the non-uniformity is 3.8% and the collimation angle is 3.2°.