A method of modeling the degradation of GaInP/GaAs/Ge triple-junction solar cells subjected to the harsh space environment is proposed in this paper. By analyzing the in-orbit data, the output current is selected as the crucial performance parameter to describe the degradation of solar cells. A mathematical model is established to model the variation tendency of the output current, and then the lifetime prediction of 3J solar cells is conducted. Most importantly, after the model precision verification, the current values obtained from the calculations show good agreement with telemetric current values with an accuracy of 0.65%. These results guarantee that the degradation modeling method developed in this study is effective for the lifetime prediction of GaInP/GaAs/Ge 3J solar cells. The fits with experimental data are correct in all cases, thus illustrating the generic character of the method it applies to modern cells, single, double or triple junctions made of GaInP, GaAs and Ge materials. (C) 2015 Elsevier Ltd. All rights reserved.