The two-dimensional thermoelectric models of single-junction and triple-junction GaAs solar cells are established respectively utilizing Sentaurus-TCAD, to investigate the damage effects caused by HPMs. Simulation results demonstrate that there are two burnout mechanisms of GaAs solar cells: damage caused by Joule heat accumulation under high electric field, and failure due to temperature surges induced by avalanche breakdown. In addition, fitted empirical formulas also show that burnout caused by Joule heat accumulation at the inflection point of the cathode front surface field occurs when the frequency of injection is above 3GHz, and damage energy decreases as the frequency increases. In contrast, the avalanche multiplication effect in the reverse-biased space charge region near the back-surface field can be triggered when the frequency is below 3GHz, and damage energy rises as the frequency rises. Besides, due to the enhancement of heat dissipation and the drop in avalanche ionization rate, the multi-junction GaAs solar cell becomes more difficult to burn out than the single-junction solar cell under the same HPM interference. Moreover, an equivalent model (based on the carrier mobility distribution when the injected HPMs signal does not reach the burnout threshold) is rebuilt to study the soft damage effect on the performance of GaAs solar cells, as caused by the injection of HPMs.