As a typical metal-free layered organic semiconductor photocatalyst, carbon nitride suffers from low light harvesting ability and unsatisfied photocatalytic activity ascribed to insufficient optical absorption and strongly bound exciton. In this paper, we reported the fabrication of a well crystallized carbon nitride with superior visible-light-driven photoreactivity by simply polymerization of dicyandiamide in a closed stainless steel autoclave which is used to afford a high pressure reaction environment. High pressure induced polymerization of dicyandiamide not only prevents the emission of hazardous gas, improving the yield of carbon nitride, but also enhances the crystallization of carbon nitride. The 550 degrees C calcined sample at high pressure (HP550) extends the interplanar packing distance of layered carbon nitride from 0.676 nm to 0.697 nm while compacts the layered stacking distance from 0.327 nm to 0.322 nm when compared its counterpart that was synthesized under other identical conditions but at normal pressure (NP550). HP550 exhibits the highest photocatalytic activity towards hydrogen production (772.40 umol(-1)h(-1)g(-1)) under visible light irradiation (lambda >= 420 nm), which is 7.8 times higher than that of NP550. The drastic improved photocatalytic activity of carbon nitride prepared at high pressure was attributed to the negatively shifted position of conductor band potentials, increased visible-light absorption and efficient separation of charge carriers, benefiting from the reduced pi-pi layer stacking distance and breaking of intraplanar hydrogen bonds.