The molecular structure of coke precursors deposited on the surface of aged heteropoly acid catalysts in alkylation is determined by various analysis methods, such as IR, NMR, GC-MS, and GC. The physicochemical and surface properties of coke precursor are characterized by BET adsorption, carbon analysis and TGA. It is found that the coke precursors deposited on the aged catalysts are mainly made up of long-chain alkanes, C-13, C-17, C-21 and some polynucleo-aromatic hydrocarbons. The coke precursors do not block the pores of the catalysts, but only cover the acid sites of heteropoly acid catalysts, and result in the decrease of acidic strength. It causes the selectivity of the catalysts in alkylation to decline.