The Bairendaba silver-polymetallic deposit is located in Keshiketeng County of Inner Mongolia, China. The region is an important part of the metallogenic belt on the western slopes of the southern Da Hingan Mountains. Proterozoic biotite plagiogneiss (Baoyintu Formation) and Variscan quartz diorite outcrop in the district. Based on vein cross-cutting relationships and ore structure, three mineralization stages can be recognized. These comprise: quartz-polymetallic sulfide stage, fluorite-hydromuscovite stage and calcite-sulfide stage. Arsenopyrite, pyrite, chalcopyrite, marcasite, pyrrhotite, sphalerite, galena and boulangerite are the main metal minerals, while the gangue minerals are dominated by quartz, fluorite and calcite. Petrographic, microthermometric and LRM (Laser Raman Microprobe) results of fluid inclusions in quartz from stage 1 and fluorite from stage 2, show that from stages 1 to 2, the ore-forming fluids pass from high-moderate temperature, CH4-rich fluid to low temperature water-rich fluids. The salinity of the ore-forming fluids is low salinity. Pure CH4-fluid inclusions were found in quartz from stage 1, but not in fluorite from stage 2. Integrating data on ore geology, regional mineralization background, distribution of mineral deposits and fluid inclusion results, it is concluded that the Bairendaba silver-polymetallic deposit is a Yanshanian magmatism-related, structure-controlled and moderate-low temperature hydrothermal system. Sulfur isotope composition suggests a deep source for ore-forming material.