Constructing a nanostructure with a high surface area and regulating the band gap by nonmetallic doping are two effective methods for improving the photocatalytic activity of catalysts. A green template-free synthesis strategy of S-doped g-C3N4 nanosheets is proposed via doping cystine as both the structural additive and S source. The features of S-doped samples (GCN-x%) were systematically studied, including morphology and textural and photoelectric properties, which demonstrated that the introduction of cystine and simple manipulation of the preparation process could realize self-exfoliation of g-C3N4 into nanosheets. The GCN-3% sample showed a surface area (131.88 m2·g-1) 10.7 times enlarged compared with bulk g-C3N4 (bulk-phase carbon nitride). Obvious redshift on the absorption edge induced by S doping can be observed, revealing a narrowed band gap and enhanced efficiency of photogenerated charge carrier separation. The DFT calculation results also verified that the introduced C-S site could lead to polarization of the local electric field and thus decrease the bandgap of g-C3N4 nanosheets. GCN-3% showed a 99.3% photocatalytic degradation ratio of rhodamine B in 60 min at a rate of 0.17 min-1. By scavengers experiment revealed that superoxide anion (·O2-) radicals and holes (h+) were vital active components during the photocatalytic degradation.