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Understanding the phase regulation mechanism of BaTiO 3 synthesized by a hydrothermal method is a complicated and challenging task. Here, we successfully prepared cubic and tetragonal BaTiO 3 single nanocrystals by changing the ratio of water and ethanol in the solvent. We confirm that the BaTiO 3 phase is mainly affected by the hydroxylation process due to the reaction with solvent. In particular, ethanol can be catalytically oxidized by titanium atoms, cause BaTiO 3 hydroxylation and promote the formation of a cubic phase. In the mixed solution of ethanol and water, the hydroxylation process is suppressed, which facilitates the formation of the tetragonal phase. The relationship framework between the solvent ratio, phase structure and hydroxyl defects of BaTiO 3 is established. Tetragonal BaTiO 3 with fewer hydroxyl defects can promote charge transfer and surface reaction after polarization, thereby enhancing their photoelectric catalytic performance. This work provides references for the controllable synthesis of ferroelectric nanocrystals by hydrothermal methods and new insight for the utilization of polarization in photoelectrocatalysis applications. [Display omitted] • Cubic BaTiO 3 were prepared in pure ethanol or water solution due to the hydroxylation effect in the hydrothermal process. • In the mixed solution of ethanol and water, the hydroxylation process was suppressed and tetragonal BaTiO 3 were generated. • The polarization electric field of tetragonal BaTiO 3 has a significant imapct on their photoelectrocatalytic performance. [ABSTRACT FROM AUTHOR]