Utilizing solar energy to achieve artificial photosynthesis of chemical fuel is prevalent in tackling excessive CO₂ emission and fossil fuel depletion. Grievous charge recombination and weak redox capability aggravate the CO₂ photoreduction performance. Engineering tailored morphology and constructing matched heterostructure are two significant schemes to ameliorate the CO₂ photoconversion efficiency of g-C₃N₄-based composite. Herein, a novel S-scheme ultrathin porous g-C₃N₄ (UPCN)/Ag₂MoO₄ (AMO) composite was designed by in-situ growing tetragonal α-AMO nanoparticles (NPs) (5-30 nm) on UPCN nanosheets (NSs). The S-scheme charge transfer route endows UPCN/AMO with fast charge separation and strong redox capability, demonstrated by X-ray photoelectron spectroscopy (XPS), photoelectrochemical tests, steady-state and time-resolved photoluminescence (PL) spectra, and DFT calculations. The UPCN/AMO composite exhibits elevated CO₂ photoreduction performance with CO and CH₄ yield rates of 6.98 and 0.38 μmol g^-1 h^-1, which are 3.5 and 2.9 folds higher than that of pristine UPCN, respectively. Finally, the CO₂ photoreduction intermediates are analyzed, and the CO₂ photoreduction mechanism is discussed. This work provides a reference for various g-C₃N₄-based composites applied in artificial photosynthesis.