Neonicotinoid insecticides are prone to biotic or abiotic transformation processes after entering the environment, and there are few studies on the abiotic reduction and degradation process in anaerobic environment. In this paper, we discussed the process and mechanism of ferrous sulfide (FeS)-mediated reductive degradation of thiamethoxam (THI) in a simulated anaerobic environment. The results showed that FeS could effectively promote the degradation of THI in an anaerobic environment, and the effects of FeS concentrations, pH levels, and the coexistence of inorganic ions and humic acid were also investigated in the environment. Under acidic conditions, the degradation rate of THI was significantly higher than that in alkaline environment, and the presence of inorganic Al3? and NO?? ions significantly inhibited the THI degradation, while no obvious effect on THI degradation was observed in the presence of Cl-、NO3-、CO32-、K+、Ca2+、Na+、Mg2+、Fe2+、Mn2+、NH4+and humic acid. Mechanistic studies have shown that Fe(Ⅱ) and sulfur vacancies are the main reductive active sites, in which sulfur vacancies could interact with the oxygen in nitro functional groups of THI molecules to promote the electron transfer between Fe(Ⅱ) and THI. In addition, the degradation products of THI by FeS were identified, and the reduction and dechlorination degradation pathways of THI were proposed. The results of this study provide a new perspective for understanding and optimizing the natural degradation process of neonicotinoid pesticides in soil and groundwater, and provide a theoretical basis for the formulation of related environmental remediation strategies.