Graphene-like g-C3N4 nanosheets (NSs) have been successfully synthesized with a modified polymerization process of melamine by cocondensation with volatile salts. Volatile ammonium salts such as urea–NH4Cl/(NH4)2SO4/(NH4)3PO4 were added with melamine to modulate the thermodynamic process during polymerization and optimize the structure formation in situ. The surface area, surface structure, and surface charge state of the obtained g-C3N4 NSs could be controlled by simply adjusting the mass ratio of the melamine/volatile ammonium salt. As a consequence, the g-C3N4 NSs exhibited much higher activity than bulk g-C3N4 for the photocatalytic degradation of target pollutants (rhodamine B, methylene blue, and methyl orange), and it also exhibited greater hydrogen evolution under visible light irradiation with an optimal melamine/volatile ammonium salt ratio. The as-prepared g-C3N4 NSs with melamine–urea–NH4Cl showed the highest visible light photocatalytic H2 production activity of 1853.8 μmol·h–1·g–1, which is 9.4 times higher than that of bulk g-C3N4 from melamine. The present study reveals that the synergistic effect of the enhanced surface area, surface structure, and surface charge state is the key for the enhancement of photocatalytic degradation and hydrogen evolution, which could be controlled by the proposed strategy. The result is a good explanation for the hypothesis that adding properly selected monomers can truly regulate the polymerization process of melamine, which is beneficial for obtaining g-C3N4 NSs without molecular self-assembly. Considering the inexpensive feedstocks used, a simple synthetic controlling method provides an opportunity for the rational design and synthesis, making it decidedly appealing for large-scale production of highly photocatalytic, visible-sensitizable, metal-free g-C3N4 photocatalysts.