In this work, we systematically investigate the photocatalytic mechanism of g-C₃N₄/BiOI (001) through hybrid functional calculations based on first-principles theory. The staggered band structure is observed in the g-C₃N₄/BiOI (001); meanwhile, a built-in electric field exists from the g-C₃N₄ monolayer to the BiOI surface at the interface. BiOI has lower band edges, which bend downward at the interface; whereas g-C₃N₄ has higher band edges, which bend upward. With Coulomb interaction and the built-in electric field, photo-generated electrons in the conduction bands (CB) of BiOI recombine with photo-generated holes in the valence bands (VB) of g-C₃N₄. Meanwhile, the stronger reduction capacity for photo-excited electrons in the g-C₃N₄'s CB and the stronger oxidation capacity for photo-generated holes in the BiOI (001)'s VB are retained. Therefore, a direct Z-scheme heterostructure character is presented. As a result, the electrons and holes generated by the photons can be separated and migrate highly effectively at the interface. The separated electrons and holes can effectively participate in the redox reactions with water/pollutants to produce the photocatalytically reactive species superoxide ions (˙O₂⁻) and hydroxyl radicals (˙OH), respectively. This is consistent with the experimental results. It is also worth noting that the g-C₃N₄/BiOI (001) heterostructure shows a larger difference in the effective mass of carriers. Therefore, the direct Z-scheme charge transfer and separation mechanism and the larger effective mass difference of carriers lead to the superior photocatalytic activity of the g-C₃N₄/BiOI (001) in experiments. A few speculations and controversies that arose from the experiments are clarified.