Inverted metamorphic triple junction (IMM3J) GaInP/GaAs/InGaAs solar cells have the advantages of high efficiency, excellent radiation resistance, lightweight and flexible properties, especially suitable for space application. In this paper, we first fabricate the IMM3J GaInP/GaAs/InGaAs solar cell, which has a short circuit current density of 16.5 mA/cm2, an open circuit voltage of 3141.8 mV, a fill factor of 84.3%, and an efficiency of 32.2%. Then, the IMM3J solar cell is irradiated by 2 MeV protons with different fluences from 2 × 1011 cm−2 to 2 × 1012 cm−2. Finally, the output electrical properties of IMM3J solar cells at the beginning of life and end of life are analyzed by current-voltage characterization. The degradation behaviors of each subcell before and after irradiation can also be described by the external quantum efficiency and short circuit current density.