An improved illumination is developed to reduce the false acceptance rate of the existing steel ball surface inspection system. A mathematical model was established to help finding out the factors having influence on the illumination uniformity. And Lighttools simulation was applied to simulate the influence of the light source shielding screen shape and guide rail transmittance on illumination uniformity. In our experiment, a new illumination device based on the analysis of simulation performance was designed and implemented into the inspection system. The result shows that the improved illumination reduces the false acceptance rate to 0.02% and increases the inspection accuracy. The improved illumination meets the theoretical derivation and expectation of computer simulation.