Phosphotungstic acid (H3PW12O40, HPW)/bimodal mesoporous silica (BMS) multifunctional catalysts were prepared and employed as photocatalyst for perfluorooctanoic acid (PFOA) degradation under vacuum ultraviolet light (VUV). Lower PFOA concentration, appropriate HPW/BMS dosage (0.2 g center dot L-1) and acidic condition (pH = 3-4) were more favorable for PFOA defluorination in VUV/HPW/BMS system. Furthermore, we analyzed the intermediates produced during PFOA decomposition by HPLC/MS/MS, which were shorter-chain perfluorinated carboxylic acids (PFCAs) with C-7, C-6, C-5, C-4, C-3 and C-2. The defluorination mechanism of PFOA was proposed, i.e., direct photolysis played an important role in PFOA defluorination under VUV radiation (lambda = 185 nm), while the defluorination of PFOA was simultaneously photocatalytically decomposed and defluorinated in the presence of HPW/BMS (lambda = 254 nm).