In this paper we characterize the microstructures of Mo/si multilager in space solar telescope induced by proton irradiation by transmission electron microscope (TEM). The optical performances of the Mo/Si multilayer mirror before and after proton irradiation are also determined. The experimental results show that some defect structures are introduced after proton irradiation. The Mo/Si periodic structure is destructed in some irradiation regions. The widths of Mo layer and Si layer significantly change and the Mo/Si interface is much rougher than before irradiation. Moreover, the obvious distortions of the Mo/Si interface and the formation of the small islands in Mo layer are also found. Furthermore, proton irradiation reduces the reflectivity of Mo/Si multilayer mirror. The dominante mechanism of optical degeneration of the Mo/Si multilayer induced by proton irradiation should be attributed to the formation of the defect structures.