In this study,complete tetracycline (TTC) and above 50％ of total organic carbon (TOC) were removed by FeS/PS after 30 min under optimized conditions.Although free radicals and high-valent iron ions were identified to generate in the process,the apparent similarity be-tween intermediate products of FeS/PS,Fe/PS,and UV/PS systems demonstrated that the degradation of TTC was due to sulfate radicals (SO4·-) and hydroxyl radicals (· OH).Based on the reaction between free radicals and organic matter,we speculated that TTC in the FeS/PS system was decomposed and mineralized by dehydration,dehydrogenation,hydroxyl addi-tion,demethylation,substitution,E-transfer,and ring-opening.Furthermore,a new under-standing of FeS-mediated PS activation based on stoichiometry and kinetic analysis showed that there were both homogeneous and heterogeneous reactions that occurred in the entire progress.However,due to the effect ofpH on the dissolution of iron ions,the homogeneous reaction became the principal process with iron ions concentration exceeding 1.35 mg/L.This work provides a theoretical basis for the study of the degradation of TTC-containing wastewater by the iron-based advanced oxidation process.