In order to investigate the characteristics of the back-side surface, i.e., Al/Si, of graphene/Si solar cells, a ultra-thin magnesium oxide interlayer with various thickness between the Al electrode and the silicon substrate was prepared by using a successive thermal evaporation deposition technology. The current-voltage curves, external quantum efficiency, series resistance, and back contact resistance of the graphene/Si solar cells with different thickness of the magnesium oxide interlayer were measured. The results show that the photoelectric conversion efficiency, series resistance and back contact resistance increased first and then decreased with the increase in the magnesium oxide thickness, and the device performance is best when the thickness of magnesium oxide layer is 1 nm. The photoelectric conversion efficiency, series resistance and back contact resistance of the solar cells with 0 nm and 1 nm MgO are 2.90%, 4.2 Ω, 9.6 Ω, and 5.53%, 1.8 Ω, 3.2 Ω.