For a thorough understanding of graphene/GaAs heterojunction solar cells, the effect of the graphene work function, absorption loss, density of interface states, as well as the incorporation of an AlxGa1-xAs (0 <= x <= 1) buffer layer on the performance of graphene/GaAs heterojunction solar cells was investigated via AFORS-HET simulation software. It is demonstrated that graphene with a work function of approximately 5 eV favors the efficiency improvement. The absorption loss is also an unnegligible factor for the optimization of the graphene work function. The density of interface states can significantly affect the carrier recombination at the heterojunction interface between graphene and GaAs and the corresponding photovoltaic performance. In addition, the work function of a back electrode should also be taken into account for selecting a back electrode material due to its significant influence on the carrier transportation and collection. Furthermore, when an AlxGa1-xAs (x = 0.2) buffer layer was introduced to this heterojunction interface, an enhanced photovoltaic conversion efficiency of 24.78% can be achieved. Finally, a comparison among simulation and experimental data for the graphene/GaAs heterojunction solar cells is presented. This study indicates the great potential of graphene/GaAs heterojunction for the application in high-efficiency solar cells.