This study quantitatively evaluated the effect of chloride (Cl) on the radical distribution and micropollutant degradation in the UV/peroxydisulfate AOP using both experimental and modeling approaches. Results showed that SO was significantly scavenged by Cl at environmentally relevant concentrations (1-5000 mg/L). With increasing Cl concentrations from 1 to 5000 mg/L, Cl transformed SO to HO and then to Cl. The critical role of Cl as a precursor of HO in the radical transformation was highlighted. The inhibitory effects of bicarbonate and dissolved organic matter (DOM) on micropollutant degradation was more significant in the presence of Cl than that in the absence of Cl, mainly due to the consumption of Cl by bicarbonate and DOM. Using the model-predicted radical concentrations in the UV/peroxydisulfate process in the presence of different concentrations of Cl, the degradation rate constants of 34 micropollutants and the contributions of each radical to the degradation were predicted and compared. The findings improved the fundamental understanding of the Cl effect on radical transformation and micropollutant degradation in the SO-based AOPs. The model enables to foresee whether a SO-based AOP is effective for the degradation of a certain micropollutant in the water with known concentrations of Cl.