Though gallium arsenide (GaAs) solar cells are proven to be relatively stable in space working conditions, they are prone to the effects of aging, which deteriorate their characteristics. The lifetime of solar cells is restricted by the degree of radiation damage that they receive. This important factor affects the performance of solar cells in practical applications. The aim of this article is to investigate by numerical simulation on the influence of aging on the main characteristics of GaAs solar cells in the space. Degradations of the electrical characteristics are simulated for over a period of 15 years. The atmosphere (AM0) conversion efficiency decreases with time from 19.08% for the unirradiated cells to 10.38% in 15 years of the mission in space. Even with low doses of particle irradiation, the performance is significantly reduced subsequent to usage over the period of 15 years of the mission in space. Numerical simulation results also reveal that the short-circuit current, the open-circuit voltage, and the conversion efficiency decline gradually with time. Moreover, the calculated evolutions are in good agreement with the measured behaviors of GaAs-based solar cells embedded in geostationary satellites during the Navigation Technology Satellite 2 (NTS-2), the Engineering Test Satellite V (ETS-V), and the NAVigation Satellite Timing And Ranging (NAVSTAR) missions, which substantiate the introduced aging law accounting for both the cumulated doses of particles and the different electron and hole traps in the structure.