This research was designed for the first time to investigate the activities of photocatalytic composite, Ag₃PO₄/g-C₃N₄, in converting CO₂ to fuels under simulated sunlight irradiation. The composite was synthesized using a simple in situ deposition method and characterized by various techniques including Brunauer–Emmett–Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and an electrochemical method. Thorough investigation indicated that the composite consisted of Ag₃PO₄, Ag, and g-C₃N₄. The introduction of Ag₃PO₄ on g-C₃N₄ promoted its light absorption performance. However, more significant was the formation of heterojunction structure between Ag₃PO₄ and g-C₃N₄, which efficiently promoted the separation of electron–hole pairs by a Z-scheme mechanism and ultimately enhanced the photocatalytic CO₂ reduction performance of the Ag₃PO₄/g-C₃N₄. The optimal Ag₃PO₄/g-C₃N₄ photocatalyst showed a CO₂ conversion rate of 57.5 μmol·h^–1·g_cat^–1, which was 6.1 and 10.4 times higher than those of g-C₃N₄ and P25, respectively, under simulated sunlight irradiation. The work found a new application of the photocatalyst, Ag₃PO₄/g-C₃N₄, in simultaneous environmental protection and energy production.