Effect of phosphate on pollutant removal was widely investigated in various peroxumonosulfate-based advanced oxidation processes (AOPs) but the role of phosphate was controversial. Many studies also used phosphate as buffer in PMS-based AOPs. This study shows that phosphate promoted PMS activation for pollutants removal. Therefore, PMS activation capacity of some strategies may be overestimated when conducted in the phosphate buffer. Importantly, the results of radical quenching tests, radical probes and reaction stoichiometries indicated that the yields of SO 4 − was elevated by the presence of phosphate and linearly depended on phosphate concentration, while the yields of OH remained constant. Density Functional Theory (DFT) calculation confirmed that PO 4 3− and HPO 4 2− could efficiently break O O bond of PMS, promoting the generation of SO 4 − for contaminants decomposition. These results illuminated the role of phosphate in AOPs utilizing PMS and uncovered the underlying mechanism of PMS activation by phosphate.