Trimethylolpropane (TMP) tricaprylate was prepared by using trimethylolpropaneand octanoic acid as raw materials and acidic ionic liquid as catalyst．The effect of the reactants feed ratio，reaction time and amount of catalyst on reaction was studied. The results showed that the optimum reaction conditions were as follows：molar ratio of acid to T-MP is 4.2:1 and catalyst amount is 9 percent of the reactants，the reaction temperature is 140℃，and reflux for 2h．Under this condition，the yield of trimethylolpropane tricapryla-te is up to 98％.The reaction system exhibited good biphasic behavior when the reaction was completed．There is no appreciable decrease of catalyst activity for both the systems after 5 cycle catalyst repetition. The product Was analysed by IR, LCMS, HPLC, NMR.