Defect engineering and the high index surface (HIF) can be regarded as two effective methods to tuning the microstructure of catalysts and photocatalytic activities. So far, the integration of the synergistic effects of oxygen vacancies and HIF advantages into semiconductors to enhance photocatalytic performance has received little attention. Herein, Black In₂O_3−x/In₂O₃ with oxygen vacancy and exposed (321) surface active crystal plane has been synthesized as excellent photocatalysts via a facile method. The catalysts of Black In₂O_3−x/In₂O₃ can reach the highest photocatalytic hydrogen evolution rate (1046 μmol h⁻¹ g⁻¹) and degradation efficiency of MB (0.017 min⁻¹) with 120 min, respectively. As a special phase junction, Black In₂O_3−x/In₂O₃ could effectively boost the separation and transfer of photogenerated charges because of unique defect homojunction microstructure resulting in exposing abundant photocatalyst redox active sites. The experiment characterization and density functional theory (DFT) calculations can further employ to unveil mechanism of enhanced hydrogen evolution reaction (HER) activity. It is obvious that enhanced HER activity was attributed to synergy of oxygen defect and exposed (321) surface active crystal planes due to electron distribution and lower Gibbs free energy of H adsorption. This work might open up new avenues to integrate exposed facets and oxygen vacancy for enhancing the photocatalytic hydrogen evolution with renewable energy sources and environmental remediation.