In order to study the degradation behavior of the related parameters of GaAs sub-cells induced by space irradiation, the proton irradiation simulation was carried out under different irradiation conditions, taking GaAs sub-cells of three-junction solar cell as the research object. The structure model of GaAs sub-cells was established and the degradation results of short-circuit current, open-circuit voltage, fill factor and maximum power induced by proton irradiation with different energies and fluences were obtained. The normalized maximum power degradation of GaAs sub-cells induced by proton irradiation versus fluence was verified by the existing experimental data. According to the maximum power degradation results of GaAs sub-cells under different irradiation conditions, the degradation equation of normalized maximum power versus displacement damage dose was obtained. The research results indicate that the irradiation defects induced by proton irradiation directly lead to the degradation of sub-cells, and the short-circuit current, open-circuit voltage, fill factor and maximum power of GaAs sub-cells degrade with the increase of proton fluence. When the proton fluence is more than 1×10 ¹¹ cm ^-2, the degradation degree of the normalized electrical parameters of GaAs sub-cells is directly proportional to the log value of proton fluence. Meanwhile, the degradation of the electrical parameters increases gradually with the decrease of proton irradiation energy. The degradation of the external quantum efficiency of GaAs sub-cells induced by proton irradiation in the long-wavelength range is more serious than that in the short-wavelength range.