As an emerging pollutant, Diclofenac (DCF) has potential threats to ecosystem and human health, and it can hardly be removed by conventional wastewater treatment processes. In this study, Fe(II)-activated bisulfite (BS), an advanced oxidation process, was used for rapid removal of DCF. The effect of initial pH, Fe(II) dosage, BS concentration, dissolved oxygen and reaction temperature on DCF removal and its degradation mechanism were investigated. Compared to Fe(II)/persulfate system, the removal efficiency of DCF was higher by Fe(II)/BS, and its degradation followed pseudo-first order kinetic model. Due to the morphology of Fe(II) and BS, the optimal pH for DCF degradation was 4.0. The increased initial Fe(II) or BS concentration promoted DCF degradation while excess Fe(II) or BS caused an inhibition effect as a SO4- scavenger. Dissolved oxygen was an essential factor inducing the conversion of SO3- to SO4-, while it had no effect on DCF removal in the range of 4.6-8.3 mg L-1. The activation energy of this reaction was calculated to be 120.75 ± 3.43 kJ mol-1 based on the improved DCF degradation with increasing temperature. According to the radical scavenging experiments, the contribution of SO4-, HO and the other reactive species to DCF degradation in Fe(II)/BS system were 71.1%, 24.6% and 4.3%, respectively. Nine transformation products were detected using UPLC-Q-TOF-MS. The potential degradation mechanism of DCF was thus proposed showing five reaction pathways including hydroxylation, decarboxylation, dehydration, dechlorination and formylation.