The present invention relates to film preparing technology, and is especially one active atom beam spraying DC discharging process for preparing nanometer carbon nitride film. One set of arc heated plasma beam source is utilized to decompose methane and nitrogen to obtain outgoing particle beam comprising neutral nitrogen atoms and methyl radicals in the strength of 1019-1020 atoms/rad.sec. The outgoing particle beam reaches to the substrate to synthesize carbon nitride film at low substrate temperature. The synthesized film consists of compact 21-50 nm size spherical crystal grains of beta-C3N4, graphite phase C3N4 and CNx mainly.