In this paper, we simulated part of the low-Earth orbits space radiation environment to investigate its influence on the reflectivity change of Mo/Si multilayer mirror used in the space solar telescope. The reflectivity was investigated before and after irradiation with proton of different energies and fluences. The results show that the reflectivity decreases when irradiated by proton because of the radiation-induced damage in the multilayer mirror, and it drops more when the energy of the proton decreases while their fluence increases. When the energy of proton E=160keV with fluence　=6×1011/mm2，E=100keV with fluence　=6×1011/mm2 and E=50keV with fluence =8×1012/mm2，the reflectivity drops about 4.1%, 5.7% and 10.4%， respectively. The roughness measured by atomic force microscopy increases after irradiation, which results in more scattering of incident ray so that the reflectivity of multilayer is reduced.