Abstract: Epoxidized soybean oil （ESO） was prepared without solvent or sulfuric acid. Some factors affecting the epoxy value of product，such as the type of carboxylic acid，amount and concentration of hydrogen peroxide and so on were investigated. Compared with acetic acid and crylic acid，formic acid had higher activity for epoxidation. The synthesis process was optimized by orthogonal experiment，that is，m(soybean oil)︰m(formic acid)︰m(hydrogen peroxide) = 1︰0.15︰1.0，reaction temperature：60 ℃，reaction time：5～6 h. The epoxy value was above 6.20% and residual iodine value was below 6.0%.The product was characterized by FT-IR. The absorption peak of ethylenic linkage of the raw material at 3008 cm-1 disappeared，instead the characteristic absorption peak of stretching vibration for the epoxy ring appeared at 820 cm－1 and 787 cm－1.