The experiment of GaAs/Ge solar cells co-irradiated by proton and electron was done in space environment simulation equipment. The materials of GaAs/Ge solar cell are irradiated by proton beam with fixed energy of 100keV and dose from 1×109 to 3×1012cm-2. The I-V characteristic，spectral response and photoluminescence (PL) spectra were measured before and after irradiation in order to study the cells performance degradation induced by irradiation. The result indicated that the parameters such as short circuit current (Isc)，open circuit voltage (Voc)，maximum output power (Pmax) and fill factor (FF) all suffer degradation at different degrees as the irradiation fluence increases. The results show that proton irradiation induces a great damage in optical characteristics of the solar cell，resulting from the large quantity of irradiation defects that would destroy crystal lattice integrity and reduce the diffusion distance of minority carrier，and thus increase the surface recombination velocity. The damage extent of GaAs/Ge solar cell increases with proton dose in the ranges under investigation.