Here　we　realize　the　decoration　of　isolated　platinum　atoms　onto　the　surface　step　of　CdS　nanowires.　The　single-atom　co-catalyst　with　high　stability　can　realize　maximum　atom　efficiency　and　significantly　boost　electron-hole　separation　efficiency　in　chromophore　units,　generating　a　greatly　enhanced　photocatalytic　hydrogen　evolution　performance,　7.69　times　as　high　as　Pt　nanoparticles　do　and　63.77　times　higher　than　that　of　bare　CdS　nanowires.　Directional　migrations　of　photogenerated　excitons　from　the　conduction　band　of　CdS　to　catalytic　platinum　centers　have　been　witnessed　by　transient　absorption　spectroscopy,　leading　to　the　supply　of　long-lived　electrons　for　highly　efficient　photocatalytic　hydrogen　evolution.　Density　functional　theory　calculations　further　confirm　that　the　excellent　catalytic　performance　is　associated　with　positively　charged　platinum　sites　with　partially　vacant　5d　orbitals,　which　change　distribution　of　charge　density　and　facilitate　higher　excited　carrier　density.