In this work, the mechanism of water oxidation catalyzed by an Ru-based complex [Ru(L)]⁺ (L = 5,5-chelated 2-carboxyl-phen, 2,2′;6′,2′′-terpyridine) was studied by density functional theory (DFT) calculations. In [Ru(L)]⁺, a carboxyl group is included in the second coordination sphere and plays an important role in the catalytic process. In the oxidation activation stage of water oxidation catalysis, the carboxyl group is proposed as a promising proton acceptor to promote proton transfer, which results in active Ru^VO species. Then, O–O bond formation can proceed via water nucleophilic attack (WNA) or oxo–oxo coupling mechanisms. In the O₂ release stage, similar to the oxidation activation process, the carboxyl group promotes proton transfer as a promising proton acceptor. In the present work, the favorable mechanism is WNA that involves proton transfer to the carboxyl group. It is expected that this work will provide meaningful information for synthesizing excellent water oxidation catalysts (WOCs).