Abstract: In order to further explore the thermal quenching effect of GaAs middle cell in electron-irradiated GaInP/GaAs/Ge triple junction solar cells, temperature dependent and excitation power dependent photoluminescence (PL) spectra for the sub-cell were measured, and the changes in PL intensity and its position were analyzed. PL intensity was fitted with temperature change using Arrhenius equation, and thermal activation energy (0.96 eV) of the non-radiative recombination center was determined. By analyzing thermal quenching effect of PL intensity in temperature range of 10-300 K, it was found that the relation between excitation power and PL intensity transforms from linear to quadratic dependence. Using the dependence, it was further verified that the main defect of GaAs middle cell is the non-radiative recombination center, and the mechanism of thermal quenching effect of electron-irradiated GaAs middle cell was explored.