In order to improve the efficiency of epoxidation，the environmentfriendly plasticizer was synthetized by catalytic epoxidation of soybean oil in solventfree system when deep eutectic ionic liquid was used as the acidic phase transfer catalyst，with choline chloride ( ChCl) being a hydrogen bond acceptor and oxalic acid dihydrate in molar ratio of 1∶ 1 and peroxyformic acid being oxygen supplying agent． The effects of reaction temperature，reaction time，catalyst，hydrogen peroxide，and formic acid dosage on epoxide yield of raw oil by the single factor experiment were investigated． The product prepared under the optimal conditions had light color with the high epoxide yield of 90. 32% and epoxy value of 6. 85%． The quality of product conformed to Chinese national standard， and the catalyst was easily separated and recycled． The characterization results from Fourier transform infrared spectroscopy ( FTIＲ) and nuclear magnetic resonance spectroscopy ( 1H NMＲ) showed that the conversion of unsaturated double bonds in raw material oil was basically complete with the high epoxide yield and excellent selectivity; thermal analysis ( TG-DSC) proved the prepared epoxidized plasticizer along with the initial decomposition temperature of 265℃ formed the trend to lose its weight until temperature was just above 223℃，respectively，displaying the better thermal stability