A novel plasmonic photocatalysts of Ag-AgI/Bi3O4Cl was successfully synthesized via a simple and mild wet-chemical process. The 30 wt% Ag-AgI/Bi3O4Cl photocatalyst showed the highest photocatalytic activity for degradation of methyl orange (MO) under visible light (5 W LED lamp), which represents a 3.2 and 7.4 times enhancement in the photocatalytic degradation efficiency as compared to pure Bi3O4Cl and Ag-AgI, respectively. The surface plasmon resonance (SPR) effect of Ag nanoparticles (NPs) was detected by the UV vis diffusion reflectance spectroscopy (DRS) and surface photovoltage (SPV) measurement, finding that the SPR effect of Ag extends the absorption and photo-response range in the Ag-AgI/Bi3O4Cl composite photocatalyst. Based on the analysis of electron spin resonance (ESR), linear sweeping voltammetry (LSV) characterization results, three electron transfer processes were proposed: AgICB -> Bi3O4ClCB, AgICB -> Ag and methyl orange -> Ag. Such electron transfer paths can not only effectively improve the separation efficiency of photo-generated carriers, but also ensure the stability of the catalyst. This study can provide a novel insight for modification of other desirable semiconductor materials to achieve high photocatalytic activity.