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 H2 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.