Efficient co-catalysts are crucial for photocatalytic water oxidation towards conversion of solar energy into chemical energy. Herein, we develop a sustainable and effective water oxidation system using graphitic carbon nitride (g-C3N4) and cubic cobalt manganese spinel (c-CoMn2O4) as light transducer and water oxidation co-catalyst, respectively. The surface modified g-C3N4 with c-CoMn2O4 not only accelerates the interface transfer rate of charge carriers but also reduces the excessive energy barrier for O-O formation, leading to an enhanced photocatalytic activity of water oxidation. Benefiting from the well surface engineering of g-C3N4, the g-C3N4-CoMn2O4 (CN-CM) composites exhibit an enhanced performance of photocatalytic water oxidation. The oxygen evolution rate (OER) of CN-CM is 4 times higher than that of pristine g-C3N4. It is a highly active in oxidation of water, with an apparent quantum yield (AQY) of ca. 1% at 380 nm with AgNO3 as sacrificial agent. This improvement is mainly due to the mixed-valence Co and Mn cations contained in c-CoMn2O4 spinel.