Abstract: An acidic functional ionic liquid 1-(3-sulfonic group)propyl-3-methylimidazole dihydro-phosphate((HSO3-pmim)H2PO4) was prepared and characterized by FT-IR, 1H NMR and 13C NMR. Hydration of α-pinene was studied over the composite catalytic system of sulfonic alkylimidazole phosphate-chloroacetic acid((HSO3-pmim)H2PO4-ClCH2COOH). The optimum conditions for hydration of α-pinene were obtained as follows: n(α-pinene):n((HSO3-pmim)H2PO4):n(chloroactic acid):n(water) 6:1:6:30,reaction temperature 80℃, reaction time 8 h. Under the above conditions, the conversion of α-pinene was 97% and the selectivity of terpineol was 47.1%. The reusability of the composite catalytic system was investigated. Under the optimum conditions, when the composite catalytic system was repeatedly used for 5 times, α-pinene conversion reached 83.7% and the selectivity of terpineol was51.6%. It also can be found that the simple experimental and product-isolation procedure combinated with easy recovery and reusability of the composite catalytic system are expected to contribute to the development of clean and environmentally friendly strategy for the synthesis of α-terpineol.