The recent thrust in utilizing g-C3N4 as a metal-free photocatalyst has led to accelerated attention in environmental decontamination. Despite of enormous efforts paid on material modification, the overall efficiency is still limited, especially for decomposition of chemically stable contaminants and mineralization of organics. Here, we coupled bulk g-C3N4 (GCN-T and GCN-D synthesized from thiourea and dicyandiamide, respectively) photocatalysis with ozonation for mineralization of oxalic acid (OA) and p-hydroxybenzoic acid (PHBA) under UV and visible-light irradiation. The compelling experiments confirmed that bulk g-C3N4 could trigger a super synergy between photocatalysis and ozonation under visible light. The apparent rate constant of OA removal by Vis/O3/GCN-D was 20.6 times as great as the sum of that in Vis/GCN-D and ozonation. The TOC removal of PHBA by Vis/O3/GCN-D was 98.0%, which was about 39.3% higher than the sum of that in Vis/GCN-D and ozonation. The high conduction band (CB) position of g-C3N4 proved to be crucial for the synergy between photocatalysis and ozonation. GCN-D possessed a ~0.10V upshifted CB level compared to GCN-T, resulting in a 1.30 times higher photocatalytic ozonation activity. More negative CB potential benefited photoinduced electron capture by ozone molecules, thus significantly enhanced charge separation as well as the decay of ozone eventually with generation of abundant hydroxyl radicals (OH). Exceptionally, Vis/O3/GCN-D exhibited stronger oxidizing ability than UV/O3/GCN-D for OA degradation with the same light intensity. The reason was that the photolysis of ozone occurred under UV and partly inhibited ozone capturing photoinduced electrons. The amount of generated OH thus decreased in UV/O3/GCN-D instead with the formation of a few atomic oxygen species (O (1D)). This study puts forward new application of the easily obtained bulk g-C3N4 and may drive the advance of sunlight/O3/g-C3N4 method for efficient and green water decontamination using sunlight as energy source.