Deep eutectic solvents based on choline chloride-carboxylic acids, as a low-cost effective bifunctional catalyst, were successfully used to catalyze epoxidation of soybean oil with peroxyformic acid as the oxygen supplier under solvent-free conditions. The influence of reaction temperature and time on the epoxidation process evaluated by H-1 nuclear magnetic resonance (NMR) quantitative analysis was extensively investigated. The optimal mild conditions were 50 degrees C and 8 h for choline chloride-oxalic acid dihydrate with 88.80% conversion of double bonds and a high selectivity of 93.86%. Meanwhile, choline chloride-malonic acid at 50 degrees C for 10 h exhibited better catalytic performance, 90.87% selectivity versus only 83.52% in the choline chloride-citric acid monohydrate system for 8 h. The epoxy products analyzed simultaneously by H-1 NMR and the titration method were characterized in Fourier transform infrared, C-13 NMR, and thermogravimetric analysis. The results suggested choline chloride-oxalic acid dihydrate with a favorable recydability was more efficient for the promotion of epoxidation of soybean oil.