For improving the photocatalytic activity, a new N-TiO2−X/g-C3N4 (NTCN) composite was designed and constructed successfully by a facile hydrothermal-calcination method. The co-doping N and Ti3+/Ov were achieved to reduce the band gap of TiO2, and g-C3N4 nanosheets were coated firmly on the surface of N-TiO2−X to form efficient heterojunctions. The photocatalytic activity of N-TiO2−X/g-C3N4 was evaluated fully under visible light irradiation to achieve the degradation of 2,4-dinitrophenylhydrazine (2,4-DNPH). When the content of g-C3N4 in composite is 30 wt% (NTCN0.3) with calcination temperature of 350 ℃, the photocatalytic degradation rate of 2,4-DNPH can reach 93.19% after 70 min irradiation, which is much higher than those of monomers and other composites. Furthermore, the degradation process was verified to be pseudo-first-order kinetic model, and the apparent activation energy is 17.23 ± 2.47 kJ mol−1. S-scheme heterojunction mechanism was speculated to explain the extremely high photocatalytic degradation ability of NTCN0.3 composite.