g-C3N4　photocatalysts　were　synthesized　by　copolymerization　of　diaminomaleonitrile　(DMNA)　with　dicyanodiamide　(DCDA)　at　high　temperatures.　The　effect　of　copolymerization　on　the　crystal　structure,　chemical　structure,　band　structure,　texture,　optical　property,　and　photocatalytic　performance　of　g-C3N4　was　studied　by　such　characterization　techniques　as　X-ray　diffraction　(XRD)　patterns,　Fourier　transformed　infrared　(FTIR),　transmission　electron　microscopy　(TEM),　nitrogen-sorption　(N-2-sorption),　electron　paramagnetic　resonance　(EPR),　ultraviolet-visble　diffuse　reflectance　spectra　(UV-Vis　DSR),　and　photoluminescence　(PL)　analyses.　Results　demonstrated　that　the　graphitic-like　layer　packing　structure　of　g-C3N4　remained　unchanged　after　the　modification;　however　the　copolymerization　with　DMNA　can　efficiently　extend　the　delocalization　of　pi-electrons　and　induce　the　formation　of　surface　junctions,　greatly　enhancing　the　light-harvesting　ability　of　g-C3N4　in　visible　light　region　and　promoting　the　separation　of　photogenerated　charge　carriers,　respectively.　Photocatalytic　performance　showed　that　all　DMNA-modified　samples　presented　an　enhanced　H-2　evolution　activity　under　visible　light　irradiation.　The　optimized　weight-in　amount　of　DMNA　is　found　to　be　0.01　g,　by　which　the　modified　sample　shows　the　highest　hydrogen　evolution　rate　of　45.0　mu　mol　.　h(-1).　This　value　is　4.5　times　as　high　as　that　of　the　unmodified　carbon　nitride　sample.