Photocatalytic inactivation has been proved to be one of the most promising strategies for controlling pathogenic microorganisms. Herein, a novel Ag nanoparticles (NPs) decorated sulfur doped g-C3N4 (Ag/SCN) composite was successfully prepared by combining thermal polymerization and photoreduction methods using urea and thiobarbituric acid as precursors. The as-prepared samples were characterized by XRD, SEM, TEM, XPS and UV-Vis DRS, the inactivation properties and mechanism against E. coli were investigated in depth. The results indicate that Ag NPs are uniformly and tightly decorated on the surface of SCN and the Ag/SCN composites exhibite significantly enhanced visible light response. When the decorating amount of Ag is 6%, the Ag/SCN-6 achieves the optimal photocatalytic antibacterial activity, which could completely inactivate 6.2 lg CFU·mL-1 of E. coli within 60 min of visible light irradiation. Moreover, active species trapping experiments reveal that the generated superoxide radicals (·O-2) following with holes (h+) and hydroxyl radical (·OH) dominate the photocatalytic antibacterial process.