By coupling graphene sheet and plasmonic photocatalysis technologies, a series of Ag—AgX/RGOs (X = Cl, Br, I; RGO = reduced graphene oxide) composites were prepared and found to be efficient antimicrobial agents for water disinfection upon visible light. Attributed to the efficient charge transfer by RGO sheets, the optimum Ag—AgBr/0.5% RGO could completely inactivate 2 × 107 cfu mL−1 of Escherichia coli within 8 min, much faster than bare Ag—AgBr within 35 min. The synergistic antimicrobial mechanism of Ag—AgBr/0.5% RGO was studied by Ag+ ions release evaluation, radical scavengers study, and radical determination. The enhanced photocatalytic activity of irradiated Ag—AgBr/0.5% RGO originated from the synergistic activities of its three components including Ag, AgBr and RGO, and the proposed mechanisms contained enhanced attraction by RGO followed by two pathways: primary oxidative stress caused by plasma induced reactive species like H2O2 and bactericidal effect of released Ag+ ions. Furthermore, characterization of E. coli cells using SEM, fluorescent microscopy, and cytoplasmic substance leakage illustrated that VL irradiated Ag-AgBr/0.5% RGO could not only cause metabolic dysfunction but also destroy the cell envelope and biomolecular, while irradiated Ag+ ions play a differential bactericidal action with a limited metabolic injury and no cell-membrane damage. The present work provides an efficient water disinfection technology and also opens a new idea in studying the antimicrobial mechanism of plasmonic photocatalyst.