In this work, the microstructure and optical properties of the Mo/Si multilayers mirror for the space extreme-ultraviolet solar telescope before and after 100 keV proton irradiation have been investigated. EUV/soft X-ray reflectometer （EXRR） results showed that, after proton irradiation, the reflectivity of the Mo/Si multilayer decreased from 12.20% to 8.34% and the center wavelength revealed red shift of 0.38 nm, as compared with those before proton irradiation. High-resolution transmission electron microscopy （HRTEM） observations revealed the presence of MoSi2, Mo3Si and Mo5Si3 in Mo-on-Si interlayers before irradiation. The preferred orientation such as MoSi2 with （101） texture and Mo5Si3 with （310） texture was formed in Mo-on-Si interlayers after proton irradiation, which led to the increase of thickness in the interlayers. It is suggested that the changes of microstructures in Mo/Si multilayers under proton irradiation could cause optical properties degradation.