To gain higher efficiency than that of traditional lattice-matched solar cells, newly designed up-grown metamorphic (UMM) InGaP/InGaAs/Ge solar cells were grown on Ge substrates. The defects in UMM InGaP/InGaAs/Ge can be perfectly controlled, and thus the conversion efficiency exceeds 30%. Similarly, the radiation environment in space could also cause displacement damage in UMM solar cells, causing the degradation of electrical properties. Proton irradiation at 50 keV and 150 keV was tested on the UMM solar cells. Because the top InGaP layer is thick enough, the radiation defects are limited within the top subcell without affecting the underlying subcells. In this research, 50-keV protons cause more serious degradation of short-circuit current, whereas 150-keV protons cause more serious degradation of open-circuit voltage. The difference may be related to the distribution of defects, which is calculated by The Stopping and Range of Ions in Matter (SRIM). Defects in the junction region would cause the decrease in voltage, whereas defects in the surface region could cause degradation of current. As a comparison, lattice-matched GaInP/GaAs/Ge solar cells were also irradiated by low energy protons. The degradation under 150-keV protons is more serious because the damage peak lies in the middle GaAs layer, which has poor radiation resistance. The degradation under 70-keV proton irradiation proves that a thinner GaInP layer improves the radiation resistance. It is notable that the degradation of short-circuit current and maximum power is influenced by the equivalent displacement damage dose (DDD), but this is not accurate for open-circuit voltage. The artifact signals at 700-750 nm after 50-keV proton irradiation indicate the decrease in shunt resistance of the InGaP subcell during proton irradiation.