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Ag3PO4 as a novel photoanode material, despite its arguably highest photoactivity, suffers for its poor light absorption and stability for photoelectrochemical (PEC) water oxidation. In this work, 4.5 × 4.5 cm2 Ag3PO4 microcrystalline films are grown via a room-temperature solution process, and vacuum annealing is proposed to solve the stability and light absorption issues. It is found that the major process below 400 °C of vacuum annealing is the |Recovery| process for Ag3PO4 microcrystals, when lattice defects and Ag0 surface species get reduced. Next, |Recrystallization| stage occurs at >400 °C. The recovery of native defects of silver vacancies, with both density functional theory calculation and experimental results, could simultaneously improve the light absorption and catalytic activity of Ag3PO4. The 400 °C-annealed Ag3PO4 photoanode, with enhanced light harvesting and crystal quality, exhibits 88% increase in (Jlight − Jdark) value (1.94 mA cm−2) than non-annealed photoanode (1.03 mA cm−2). Moreover, it retains >99% current density after a 4000-s stability test. These results suggest that vacuum annealing can substantially improve the PEC performance of Ag3PO4 microcrystalline film photoanodes due to mitigated effects of native defects, improved light harvesting, and inhibited Ag3PO4 decomposition during water oxidation reaction.