<p>Enhancing charge separation efficiency at heterojunction interfaces is pivotal for advancing photocatalytic CO₂ reduction. Herein, we propose a novel defect-engineering strategy to modulate the Fermi level by introducing titanium vacancies (Ti_v) into TiO₂, achieving n-type to p-type transition with a 0.64 eV Fermi level downward shift. When coupled with n-type lead-free perovskite Cs₃Sb₂Br₉, the p-type Ti_v-TiO₂ exhibits a substantial Fermi level offset (Δ<i>E</i>_F) of 1.38 eV, resulting in a 1.62-fold stronger built-in electric field in the Cs₃Sb₂Br₉/Ti_v-TiO₂ heterojunction compared to Cs₃Sb₂Br₉/TiO₂. This drives an efficient Z-scheme charge transfer that spatially separates carriers. The Ti_v sites further function as active centers, significantly lowering the water adsorption energy and accelerating water oxidation kinetics. Under simulated sunlight (100 mW cm⁻²), the Cs₃Sb₂Br₉/Ti_v-TiO₂ system achieves overall CO₂ reduction coupled with H₂O oxidation, yielding a CO production rate of 343 µmol g⁻¹ h⁻¹, which is 5.4- and 1.7-fold higher than those of Cs₃Sb₂Br₉ and Cs₃Sb₂Br₉/TiO₂ controls, respectively. This work presents a new strategy for Fermi level modulation to enhance heterojunction charge separation, offering a promising approach for efficient solar fuel production.</p>