Plasmonic photocatalyst Ag/AgCl was prepared by in situ hydrothermal method with the contribution of 1-octyl-3-methylimidazolium chloride ([Omim]Cl), in which the [Omim]Cl ionic liquid acted not only as a precursor but also as a reducing reagent in the process of formation of Ag⁰. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric and differential scanning calorimetry (TG-DSC). The photocatalytic activity of the composites were evaluated by degradation of methyl orange (MO) under visible light irradiation. The experimental results showed that the high activity and stability of Ag/AgCl photocatalysts under visible-light irradiation were due to their localized surface plasmon resonance (LSPR). Based on the characterization of the structure and photocatalytic performance, the LSPR was determined by synergetic effect of many factors, such as particle size of metallic Ag, contents of the Ag⁰ nanoparticles, and the extent of metallic Ag dispersing. A photocatalytic mechanism of the Ag/AgCl photocatalyst was also proposed.