The photocatalytic activity of graphitic carbon nitride (CN) is mainly restricted by its high recombination rate of charge carriers and narrow visible light absorption. In the present work, nitrogen-deficient CN (NDCCN) nanosheets with high crystallinity were synthesized using molten salt (NaLiCO) as an etching agent and high-temperature solvent. The electronic structure and energy band levels of the obtained NDCCN are optimized to extend its optical absorption and enhance separation efficiency of photo-generated charge carriers. With these changes, NDCCN displays high photocatalytic activity for hydrogen evolution under visible light illumination (111 μmol h), which is 4.6 times of that over pristine CN. This finding opens up a new window to simultaneously decrease nitrogen composition and increase crystallinity of carbon nitride for higher solar-light-driven hydrogen production efficiency.