Nitrogen self-doped graphitic carbon nitride (C₃N_4+x) was successfully synthesized by the co-thermal condensation of the precursor with a nitrogen-rich additive. The resultant self-doped semiconductor was characterized by X-ray photoelectron spectroscopy (XPS), which indicated that the nitrogen atom substituted the sp² carbon atom. The photocatalytic hydrogen evolution was systematically evaluated under visible light irradiation (λ > 400 nm). The average hydrogen evolution rate for C₃N_4+x was 1.8 times higher than that of pristine graphitic carbon nitride, and the superiority lay in greatly improved optical, emission and electronic properties of the nitrogen modified carbon nitride. This study filled the research gap of self-doping for 2D polymeric carbon nitride and will stimulate intensive investigations in the further improvement of photocatalytic hydrogen evolution.