Here we report a facile annealing process for homogeneous deposition of Co3O4 quantum dots (Co3O4 QDs) onto porous g-C3N4 nanosheets. It was discovered that pores were catalytically produced around Co3O4 QDs. In the synthesis, annealing temperature was found to be crucial for the textural property, optical absorption, and the corresponding photocatalytic water oxidation as well as photochemical performances. The highest specific surface area, pore volume and optimal O2 production rate as well as the highest photocurrent were obtained on 0.8 wt.% Co3O4 QDs decorated g-C3N4 nanosheets annealed at 300 °C (0.8% Co3O4-C3N4-300). These results underline the importance of surface heterojunction and afford us a feasible protocol for rational design of g-C3N4 based photocatalysts for water oxidation.