Illumination uniformity of LED arrays is studied through theoretical derivation and computing simulation. The spatial angle distribution and plane illuminance distribution are analyzed. A source model of LED arrays is designed based on the requirement of illumination uniformity of laboratory radiation calibration. The illumination uniformity of the model is simulated using LightTools. Simulation results show that the LED arrays′ irradiance uniformity is 98.3% and the spatial angle radiation uniformity is 98.2%. The Labsphere integrating sphere is tested and compared with simulation results. The simulation results are close to the test results of integrating sphere, which indicat that the designed LED arrays satisfy the requirement on source uniformity of laboratory radiation calibration.