Abiotic processes in anaerobic environments exert a significant influence on the removal and transformation of neonicotinoid pesticides. This study aims to elucidate the underlying mechanisms by investigating the reduction and degradation process of thiamethoxam (THI) mediated by ferrous sulfide (FeS) through anaerobic simulation experiments. Furthermore, the effects of various factors, including FeS concentration, environmental pH, coexisting inorganic ions, and humic acid, are systematically assessed. The results showed that FeS could effectively promote the degradation of THI in an anaerobic 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- 2 ions significantly inhibited the degradation of THI, while no obvious effect on the degradation of THI was observed in the presence of Cl^-, NO- 3, CO2- 3, K⁺, Ca²⁺, Na⁺, Mg²⁺, Fe²⁺, Mn²⁺and NH+ 4and 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.