As a metal-free photocatalyst, graphitic carbon nitride (g-C3N4) has attracted tremendous attention. Preparation of porous few-layer g-C3N4 nanosheets has been proven to be an effective strategy to obtain high photocatalytic performance. At present, most methods are expensive, time-consuming or complicated. Here, a low-cost, facile and environment-friendly one-step synthesis method of porous few-layer g-C3N4 nanosheets is designed by introducing water in the precursor. Straightforward calcination of the precursor, which decomposes to form ammonia, can produce g-C3N4 nanosheets with the assistance of water. Under the visible light (>400 nm), the photocatalytic H2 evolution performance of the so-obtained nanosheets is 3214 μmol · g-1 · h-1, which is 17.3 times of the original bulk g-C3N4. The apparent quantum yield is 27% under the 380 nm monochromatic light irradiation.