Aiming at solving the problem of the impact of earth and atmosphere radiation on star extraction accuracy of the star sensor，the model about the irradiance effect on the star sensor was proposed by adopting infinitesimal method and analysis of the variables affecting the magnitude of the irradiance were made. The mechanism of the influence of earth and atmosphere radiation was explained，and defocusing simulation of earth and atmosphere radiation with different magnitude was achieved by White Gaussian Noise (WGN) with different variance and simulation comparison of the accuracy of star extraction was made under the condition of the earth and atmosphere radiation with different position and time. The simulation results indicate that earth and atmosphere radiation can lead to the decrease of the accuracy of star extraction, which can be adopted to evaluate the anti-earth albedo performance of the lens hood of star sensor.