Recently, photocatalytic hydrogen generation from water-semiconductor catalyst dispersion attracted world-wide attention because the solar energy could be converted directly into hydrogen only in very simple set-up in scale-up scale. Although many photocatalysts were reported to be active for this reaction, the efficiency is still quite low. In this work, we found that hydrogen and oxygen recombination took place rapidly in the Pt/TiO₂ dispersion and resulted in very low hydrogen generation rate in pure water. The activation energy of H₂ and O₂ recombination reaction is 16.5 kJ/mol regardless of light irradiation or not. In this case, the photocatalytic evolved H₂ could react rapidly with the evolved O₂ to form H₂O again, eventually leading to no net H₂ and O₂ evolution during irradiation. The recombination of hydrogen and oxygen can be inhibited by addition of oxygen transfer reagent hemin chloride (HC). With help of HC, photocatalytic generated oxygen was captured by HC and transferred away from photocatalyst surface, then the backward reaction of hydrogen-oxygen recombination was successfully restrained. The photocatalytic hydrogen evolution amount in Pt/TiO₂ dispersion was significantly enhanced under this condition. The isotopes analysis results confirmed that both the H₂ and O₂ were from water. The HC could be recycled by releasing the O₂ with Ar gas bubbling. The HC could be poisoned by carbon monoxide and lost its oxygen transfer property, therefore no hydrogen could be formed under irradiation. This study clarified the main impediment reason of low efficient photocatalytic hydrogen generation in semiconductor-water dispersion and present available method to avoid this negative reaction. This finding will help to design high active catalytic system for solar energy to hydrogen conversion and open a new window for overall water splitting research.