Hydrogenated　TiO2　(H-TiO2)　has　triggered　intense　research　interest　in　photocatalysis　due　to　its　substantially　improved　solar　absorption　and　superior　activity.　However,　the　main　factors　that　induce　the　enhanced　photocatalytic　performance　of　H-TiO2　are　still　under　debate.　In　order　to　clarify　this　issue,　the　structural　properties　of　H-TiO2,　and　their　effects　on　photo-generated　charges　are　comprehensively　investigated　in　this　study.　H-TiO2　nanoparticles　with　different　hydrogenation　degrees　are　rapidly　synthesized　through　H-2　plasma　treatment　in　several　minutes;　and　their　photocatalytic　activities　are　evaluated　by　methylene　blue　(MB)　degradation　and　CO2　reduction　in　aqueous　and　gaseous　media,　respectively.　The　slightly　hydrogenated　TiO2　(s-H-TiO2)　nanoparticles　with　the　original　white　color　exhibit　enhanced　photoactivity　compared　with　the　pristine　TiO2　(pristine-TiO2)　nanoparticles　especially　for　CO2　reduction;　while　the　gray　or　black　H-TiO2　nanoparticles　with　higher　hydrogenation　degrees　(h-H-TiO2)　display　much　worse　catalytic　performances.　Further　investigations　reveal　that　the　higher　ratio　of　trapped　holes　(O-　centers)　and　a　lower　recombination　rate　induced　by　the　increase　of　surface　defects　might　be　the　critical　factors　for　the　high　activity　of　s-H-TiO2;　in　contrast,　h-H-TiO2　nanoparticles　possess　high　concentration　of　bulk　defects,　leading　to　a　significantly　decreased　amount　of　O-　centers　and　enhanced　non-radiative　recombination,　which　strongly　inhibit　their　photoactivity.　These　results　might　provide　new　insights　into　the　photoactivity　of　H-TiO2,　and　pave　the　way　for　further　studies　of　other　hydrogenated　metal　oxides　for　photocatalytic　applications.