This paper deals with the degradation of acetaminophen by Fe²⁺-activated peroxymonosulfate( PMS) in aqueous solution. By investigating the effect factors,reaction kinetics,mineralization and radical( SO^-·4or OH^·)identification,two degradation stages,one is fast and the other is slow,are discovered,and an optimal concentration ratio of PMS to Fe²⁺,namely 1. 5∶1,is determined for achieving the highest degradation rate. Moreover,it is found that the optimal pH value is 5. 0,and that relatively high PMS and Fe²⁺dosages help achieve a mineralization rate more than 55%. ESR test indicates that SO^-·4 and OH^·appear in the degradation process and play their important roles; and that SO^-·4 and OH·appear after 3 min from the reaction initiation,then the quantity of OH^·decreases and then vanishes,so that SO^-·4 is the main radical for the slow degradation stage.