Exploring efficient photocatalysts for solar driven CO₂ reduction with water (H₂O) as a proton donor is highly imperative but remains a great challenge because the synchronous enhancement of CO₂ activation, H₂O dissociation and proton transfer is hardly achieved on a photocatalyst. Particularly, the sluggish H₂O dissociation impedes the photocatalytic CO₂ reduction reaction involving multiple proton–electron coupling transfer processes. Herein, a sulfur-doped BiOCl (S-BiOCl) photocatalyst with abundant oxygen vacancies (O_V) is developed, which exhibits broadband-light harvesting across solar spectrum and distinct photothermal effect due to photochromism. For photocatalytic CO₂ reduction with H₂O in a gas–solid system, the high CO yield of 49.76 μmol·g_cat⁻¹·h⁻¹ with 100% selectivity is achieved over the S-BiOCl catalyst under a simulated sunlight. The H₂O-assisted CO₂ reduction reaction on S-BiOCl catalyst is triggered by photocatalysis and the photothermal heating further enhances the reaction rate. The kinetic isotope experiments indicate that the sluggish H₂O dissociation affects the whole photocatalytic CO₂ reduction process. The presence of oxygen vacancies promotes the adsorption and activation of H₂O and CO₂, and the doped S sites play a crucial role in boosting H₂O dissociation and accelerating the dynamic migration of hydrogen species. As a result, the ingenious integration of O_V defects, S sites and photothermal effect in S-BiOCl catalyst conjointly contributes to the significant improvement in photocatalytic CO₂ reduction performance.