We coupled g-C3N4 or chlorine modified g-C3N4 (Cl/g-C3N4) photocatalysis with ozonation for mineralization of oxalic acid (OA) under visible light. g-C3N4 and Cl/g-C3N4 could trigger a super synergy between photocatalysis and ozonation, with a coupling coefficient at 17.8 and 9.9, respectively. The great gap of redox potential between the conduction band of g-C3N4 and ozone greatly benefitted electrons captured by ozone molecules, and thus promoted charge separation and ozone self-decomposition into a growing number of hydroxyl radicals in a photocatalytic ozonation process. Besides, the influence of chlorine modification on g-C3N4 to photocatalysis and photocatalytic ozonation was also clearly stated.