The high dispersibility and solubility are highly required for the potential applications and development of well-known g-C3N4 material. In this study, a facile hydrothermal treatment and the following vacuum freezing-drying process was developed to synthesize the g-C3N4 nanosheets (ca. 5 nm) with excellent dispersibility and solubility in aqueous solutions. It was found that the melem structures with many hydrophilic groups (NH2, OH and -C=O) were formed on the g-C3N4 nanosheet surface, resulting in the formation of soluble g-C3N4 (SCN) nanosheets. Moreover, the SCN nanosheets can be worked as the effective modifier to greatly increase the H-2 production performance of conventional g-C3N4 photocatalyst (the resultant sample was referred to SCN/g-C3N4). Photocatalytic results revealed that the SCN/g-C3N4 sample exhibited a remarkably higher H-2-production performance than the pure g-C3N4 by a factor of ca. 2. The improved H-2-production rate of SCN/g-C3N4 photocatalysts could be primarily ascribed to the introduction of hydrophilic groups, which not only remarkably enhances the dispersibility and hydrophilicity of SCN/g-C3N4, but also work as the interfacial active sites to accelerate the H+-reduction reaction and the rapid formation of H-2. The present soluble g-C3N4 nanosheets provide potential various applications in environmental protection and energy conversion fields.