This work was to reduce the epoxidation time of soybean oil with the assistance of ultrasonic irradiation and a kind of fatty acid as the catalyst in the presence of formic acid as the carrier of reactive oxygen. The operational parameters affecting the epoxy value of soybean oil, such as the amount of fatty acid and the ultrasonic frequency, ultrasonic power and reaction time were studied. And the interfacial tension of the aqueous-oil phase was decreased with the catalyst fatty acid adding or the assistance of ultrasonic irradiation, therefore the reaction rate could be accelerated. The maximum epoxy value of epoxidized soybean oil was 17.19% increased compared with the epoxy value that treated by non-ultrasound assistance in the same reaction time. Experimental results indicated that the epoxy value could be enhanced under the ultrasonic irradiation assistance, where the reaction time could be effectively reduced.