Co3O4 imbedded g-C3N4 (Co3O4/g-C3N4) heterojunction photocatalysts were synthesized via initial dissolution of C, N and Co organic precursors in aqueous phase, subsequent evaporation of water and final thermopolymerization. This facile aqueous-induced complexation of organic precursors guaranteed that Co3O4 was homogeneously dispersed in g-C3N4 matrix even if the mass loading of Co3O4 reached up to 0.3–3 wt %. The as-constructed Co3O4/g-C3N4 composites were applied in visible-light-driven hydrogen evolution for the first time in which the mass loading of Co3O4 was optimized at 1 wt %, achieving a maximal hydrogen evolution rate of 50 μmol/h/g, as higher as 5 times than those of pure g-C3N4 and Co3O4. The enhanced photocatalytic activity of Co3O4/g-C3N4 composites was originated from well-established p-n heterojunctions when certain amount of p-type Co3O4 nanoparticles were introduced and highly dispersed into n-type g-C3N4 matrix. The Co3O4/g-C3N4 p-n heterojunctions effectively retard the recombination of photoinduced electron-hole pairs, promote charge separation, extend visible light absorption range and finally improve photocatalytic hydrogen evolution activity and stability. As a result, this facile, effective, green and universal strategy opens up new horizons to realize high dispersion of metal oxides in g-C3N4 matrix and to achieve higher performance in photocatalytic activity.