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Decreasing the Ti3+ defect concentration in SrTiO3 is recognized as the focus of seeking a perfect photocatalytic activity. However, the defect‐engineered SrTiO3 by doping different low‐valence metal ions have no similarly good activity in overall water spitting, of which the reason has not been determined. Here, Na+, K+, and Cs are deliberately doped in SrTiO3 to obtain a close low‐level concentration of Ti3+ defect for comparison. An interesting phenomenon is found that the K‐doped SrTiO3 has a higher Ti3+ concentration than others but with a better intrinsic photocatalytic activity. The analysis of band structure and charge‐carriers behavior indicates the formation of a deep‐state defect in Na or Cs‐doped SrTiO3, leading to a decrease in photocatalytic activity. With the density functional theory calculation and synchrotron radiation characterization, the differences in A‐site‐metal ionic polarization are found to localize some defect charge, accompanied by uneven structural relaxation. As a result, the deep‐state defect can form in the area containing some group of atoms around the higher‐polarization metal ions, limiting the activity. This study further complements the important effect of doping ions on defect state in addition to the Ti3+ defect concentration in theory, for designing defect engineering in SrTiO3. [ABSTRACT FROM AUTHOR]