The Shanmen silver deposit in Siping, Jilin Province, is located at the junction of the eastern segment of the Central Asian Orogenic Belt and the northern margin of the North China Craton, and represents the only large-scale independent silver deposit in Northeast China. However, in-situ microscopic studies on the occurrence states and enrichment mechanism of silver in this deposit are still lacking. Field geological investigation, mineragraphy, mineralogy, and in-situ sulfide geochemistry were systematically carried out to determine the occurrence states of silver and to explore its enrichment mechanism. Electron probe microanalysis(EPMA), LA-ICP-MS and LA-ICP-MS MAPPING analyses showed that silver is mainly hosted in pyrite, chalcopyrite and famatinite in the forms of sub-microscopic inclusions and isomorphism with varying enrichment degrees. Mineragraphic observations identified three types of visible independent silver minerals: polybasite, pyrargyrite and native silver. Simulation of the logfO2-pH relationships of Fe-O-S elements in sulfides indicated that the early mineralization stage was in a weakly reducing environment, where silver was mainly transported as chloride complexes. As the ore-forming fluids migrated upward, the pressure decreased sharply, leading to fluid boiling, which destabilized the chloride complexes and promoted the transformation of silver into hydrosulfide complexes (with HS-). Subsequently, the influx of meteoric water caused decreases in temperature and oxygen fugacity, reducing the HS- concentration in the fluid and triggering the precipitation of silver-bearing sulfides. Therefore, three occurrence states of silver (independent silver minerals, sub-microscopic inclusions and isomorphism) were recognized, and fluid mixing was identified as the key mechanism for silver precipitation and enrichment.