The wxAMPS software is used to simulate the GaN/Si single heterojunction solar cells, and the effects of doping concentration, thickness, and temperature of each layer on the battery open-circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (F) and conversion efficiency are investigated. The simulation results show that as the acceptor concentration in the Si layer increases, J_SC decreases, but all of V_OC, F, and conversion efficiency increase. When the doping concentration of GaN is 5×10 ¹⁸ cm ^-3 and the doping concentration of Si is 5×10 ¹⁹ cm ^-3, the conversion efficiency of the ultra-thin cell with a Si layer of 16 μm thickness can reach 16.91%. As the thickness of the Si layer increases, all of V_OC, J_SC, F, and conversion efficiency increase. When the thickness of the GaN layer is 0.005 μm and the thickness of the Si layer is 100 μm, the efficiency can reach 24.58%. The simulation results show that when the thickness of the GaN/Si single heterojunction solar cell is 60% of the thickness of the current most efficient silicon-based solar cell, the efficiency of the former can reach 92% that of the latter. The research results are helpful to fabricate high-efficiency GaN/Si single heterojunction solar cells.