Pristine Ag₃PO₄ microspheres were synthesized by a co-precipitation method, followed by being calcined at different temperatures to obtain a series of calcined Ag₃PO₄ photocatalysts. This work aims to investigate the origin of activity and stability enhancement for Ag₃PO₄ photocatalyst after calcination based on the systematical analyses of the structures, morphologies, chemical states of elements, oxygen defects, optical absorption properties, separation and transfer of photogenerated electron-hole pairs, and active species. The results indicate that oxygen vacancies (V˙˙) are created and metallic silver nanoparticles (Ag NPs) are formed by the reaction of partial Ag⁺ in Ag₃PO₄ semiconductor with the thermally excited electrons from Ag₃PO₄ and then deposited on the surface of Ag₃PO₄ microspheres during the calcination process. Among the calcined Ag₃PO₄ samples, the Ag₃PO₄-200 sample exhibits the best photocatalytic activity and greatly enhanced photocatalytic stability for photodegradation of methylene blue (MB) solution under visible light irradiation. Oxygen vacancies play a significantly positive role in the enhancement of photocatalytic activity, while metallic Ag has a very important effect on improving the photocatalytic stability. Overall, the present work provides some powerful evidences and a deep understanding on the origin of activity and stability enhancement for the Ag₃PO₄ photocatalyst after calcination.