GaInP/GaAs/Ge triple-junction solar cells have been widely used in satellite applications. The existence of high energy protons in deep space exploration will inevitably degrade the performance of the solar cells. We have conducted irradiation experiments with proton energies of 80 MeV and 100 MeV, and found that after irradiation with 80 MeV and 100 MeV protons, short-circuit current (Isc) of the triple-junction cell remains essentially unchanged, while open-circuit voltage (Voc) degrades to 93% of its value before irradiation. With technology computer aided design (TCAD) simulations, we have found that the top cell GaInP contributes the most to the overall degradation of the Voc, while the middle cell GaAs contributes the most to the overall degradation of the Isc; with 100 MeV proton irradiated on GaInP/GaAs/Ge triple-junction solar cells, the hole concentration in the top cell junction decreases from the non-irradiated level of 1.47×1011 cm-3 to 4.12×109 cm-3, leading to significant degradation of the Voc in the triple-junction cell. These results will aid in understanding the degradation behavior of characteristics in GaInP/GaAs/Ge triple-junction solar cells and their intrinsic relationship with irradiation damage in sub-cell.