Pyridinium 4-Toluenesulfonate ionic liquid (IL) was synthesized in water by a one-step synthetic method. The ionic liquid was characterized by infrared spectrum (IR), nuclear magnetic resonance spectrum (NMR) and Thermal analysis (TG-DTA). Friedel-Craft acylation of β-Methoxynaphthalene using acetic anhydride as acylation reagent was then studied as a probe reaction. The effect of catalytic activities of the ionic liquid and reaction conditions on the catalytic reaction were researched. The results indicated that, under the optimal reaction conditions, namely nacetic anhydride: nβ-Methoxynaphthalene=1.5:1, mIL : mβ-Methoxynaphthalene= 2.53, reaction temperature at 150°C for 5h, the conversion of β-Methoxynaphthalene was up to 58.88 % and the selectivity towards 2-methoxy-1-acetonaphthone reached to 98.28 %. The ionic liquid could be recycled and reused for several times without significant loss of activity. The product has been separated, purified and confirmed by infrared spectrum, nuclear magnetic resonance spectrum and thermal analysis. Two products of the reaction were identified.