As a typical visible-light-responsive organic semiconductor photocatalyst, graphitic carbon nitride (gCN) only exhibits moderate photoreactivity beacuse of its low specific area, limited light harvesting ability and quick recombination of photo-generated carriers. Herein, we report the fabrication of gCN nanosheets (gCN-NSs) with large BET surface area, excellent visible-light-absorptive property and efficient separation of carriers by direct polymerization of amidinourea, the product from the hydrolysis of dicyandiamine (DCDA). When compared with that of bulk gCN (SO) synthesized by direct polymerization of DCDA, the visible photocatalytic hydrogen evoluation rate of gCN-NSs (S2 sample) obtained by condensation of amidinourea improved 4.9, times, which is also 2.4 times higher than that of gCN-NSs (Su sample) which is prepared by polymerization of urea. The enhanced visible photocatalytic activity gCN-NSs (S2) toward hydrogen production can be ascribed to the combined effects of enlarged BET surface that provides more active sites for adsorption and photocatalytic reaction, compacted stsc stacking which facilitates the efficient separation of photo-generated carriers, negatively shifted CB potential and improved hydrophilic property that favor the electron transfer at the interfaces between gCN and water. In addition, the product yield of gCN-NSs (S2 sample) from the polymerization of amidinourea is 11.9%, which is 10.8 times higher than Su sample that prepared from urea (only 1.1%).