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SrTiO3 photocatalysts have been widely applied in overall water splitting reaction for their reasonable redox potential, whereas preventing the defects is crucial to improve the transition of photoexcited carriers influencing the photocatalytic performance. The defects in perovskite-type SrTiO3 were mainly Ti3+ and oxygen vacancies. Thence, little Ta5+ was designed to be doped in the B site for modulating the defect concentration. A series of advanced characterizations demonstrated that Ta5+ is helpful for Al3+ to penetrate the SrTiO3 lattice through lattice distortion, decreasing the Ti3+ defects. Meanwhile, the doping of slight Ta5+ facilitates the decrease in oxygen defect concentration, due to the formation of {Ta5+–O–Al3+}. Photoluminescence spectra and time-resolved fluorescence spectroscopy uncover that the charge carrier separation of SrTiO3 was enhanced by slight Ta5+. However, the excessive Ta5+ greatly promoted form the oxygen defect and Ti3+ species, leading to severe recombination of photogenerated carriers, which significantly reduced the photocatalytic activity. As a result, the optimized sample showed significant improvement in photocatalytic activity compared to the pure SrTiO3. [ABSTRACT FROM AUTHOR]