Abstract: ZnS/Al2O3/MgF2 three-layer antireflection coating with wide spectrum (300 nm～1800 nm) was designed in order to improve the photoelectric conversion efficiency of GaAs multi-junction solar cells. The influence of each layer-s thickness and refractive index on the effective reflectance of film system was analyzed. The results show that the impact of the film thickness of ZnS is greater than that of Al2O3 and MgF2 on the effective reflectance in the whole wavelength,and the impact of MgF2 is the lowest; decreasing the refractivity of MgF2 or increasing the refractivity of ZnS proportionally can get lower effective reflectance. Furthermore, when the optimal physical thickness of ZnS,Al2O3 and MgF2 is 52.77 nm，82.61 nm and 125.17 nm respectively by programming calculation,the minimum effective reflectance is 2.31％.