Plasmonic Ag nanoparticles (AgNPs) with narrow distribution were successfully loaded on graphitic carbon nitride (g-C3N4) sheet by thermal polymerization of melamine precursor and a simple wet-chemical pathway in the presence of polyvinylpyrrolidone (PVP). N,N-dimethylformamide (DMF) was used as an efficient reducing agent as well as a solvent and its presence facilitated homogeneous distribution of AgNPs under mild reaction condition and easy control of its particle growth under different precursor amounts. Ag/g-C3N4 composites of different Ag content were prepared, and the phase, chemical structure, morphologies, electronic and optical properties of Ag/g-C3N4 heterostructures were well characterized, respectively. The photocatalytic activity of Ag/g-C3N4 composites was evaluated by the decolorization of methyl orange (MO), and they exhibited superior photocatalytic activity to bulk g-C3N4 under visible-light irradiation. Influence of Ag content to photocatalytic activity was also discussed and possible mechanism was explored based on the analysis of photoluminescence spectra (PL) and photodecoloration activity.