To broaden the light response range of semiconductor photocatalysts and improve the efficiency of photocatalytic reaction are the focuses of the production of clean hydrogen energy through solar photocatalytic water splitting. Metal-free graphitic carbon nitride (g-C3N4) is an efficient visible-light-driven photocatalytic material. In this study, a kind of narrow-band-gap g-C3N4 has been synthesized at high synthesis temperature under hydrogen argon mixture. And then a two-step method of ultrasonication and calcination treatment was used to synthesize ultrathin g-C3N4 nanosheets with much bigger specific surface area. The results of characterization analysis indicate that the as-prepared ultrathin narrow-band-gap g-C3N4 nanosheets show great light absorbance in visible light and high photogenerated charge separation capability. And the as-prepared ultrathin g-C3N4 nanosheets exhibit significantly enhanced photocatalytic performance for photocatalytic hydrogen evolution under visible light (lambda > 480 nm).