Abstract: In order to optimize the UV degradation conditions of perfluorooctane sulphonate (PFOS) and understand the degradation mechanism, this article studied the removal of PFOS under 6 reaction conditions: dark reaction (Dark), dark/potassium iodide system (Dark/KI), ultraviolet (UV), ultraviolet/Potassium iodide system (UV/KI), vacuum ultraviolet (VUV) and vacuum ultraviolet/potassium iodide system (VUV/KI). Under the VUV/KI system, a single factor analysis method was used to study the effect of initial concentration, pH and KI addition on the degradation effect of PFOS. Through the analysis and identification of intermediate products, the degradation mechanism and degradation pathways of PFOS under ultraviolet light were proposed. The results showed that PFOS didn't degrade in Dark and Dark/KI systems, the degradation rate of PFOS in the UV, UV/KI, and VUV systems was less than 30%. The degradation rate of PFOS reached 70% in the VUV/KI system with initial concentration of 200 µg/L PFOS and 2 mmol/L KI in 8-hour. Decreasing the initial concentration of PFOS, increasing the pH value and the amount of KI were all helpful to the degradation of PFOS. Based on the Orbitrap LC/MS analysis, 6 kinds of intermediate products were identified: perfluorooctanoic acid (PFOA), perfluorobutyric acid (PFBA), pentafluoroethane sulfonic acid (PFEtS), methyl pentafluoropropionate (MPFA), methyl trifluoroacetate (MTFA) and trifluoroacetic acid (TFA). The degradation process of PFOS includes desulfonation and cleavage of C-C bonds. This study shows that hydrated electrons are the main active species in the degradation of PFOS by ultraviolet light, and the VUV/KI light reaction system can effectively remove PFOS. The analysis of degradation pathways and degradation mechanisms can provide references for the UV degradation of other perfluorinated compounds.