The degradation effects of the GaInP/GaAs/Ge triple-junction solar cells irradiated by <200 keV protons are investigated on the basis of the spectral response analysis and measurements of electric property. The experimental results show that with increasing proton fluence I(sc), V(oc) and P(max) decrease obviously. The proton energy exhibits an important influence on the degradation effects of the triple-junction cells dependent on the proton penetration range in the cells. As the proton energy is lower than 100 keV, irradiation-induced damage occurs in the top cell, while the irradiation with proton energy higher than 100 keV causes damage mainly in the middle sub-cells. Comparing the changes in the electrical properties of the triple-junction cells, a conclusion can be made that the GaAs middle sub-cell plays a major role in leading to more severe degradation. In this case, the 170 keV protons are suggested to be used to evaluate the performance of the GaAs triple-junction solar cells, for they can produce more severe degradation effects. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.