CdS　modified　with　reduced　graphene　oxide　(RGO)　has　been　widely　demonstrated　to　be　effective　in　the　field　of　solar-energy　conversion.　However,　the　inherent　mechanism　of　this　superior　property　is　still　not　thoroughly　understood.　Thus　the　photoelectrochemical　method　was　employed　to　systemically　investigate　the　synergetic　effect　between　CdS　and　RGO.　The　result　shows　that　the　photoelectrochemical　properties　of　RGO/CdS　samples　are　sensitive　to　the　relative　ratio　of　RGO　to　CdS,　and　the　photoelectrode　with　1.0wt%　ratio　of　RGO　possesses　the　best　photoelectrochemical　performance.　Further　investigation　demonstrates　that　the　synergetic　effect　between　CdS　and　RGO　directly　influences　the　charge-transport　property　and　band-structure　of　the　composite,　which　is　also　supported　by　the　X-ray　photoelectron　spectroscopy　data　and　first-principle　simulation,　respectively.