Abstract: Response surface methodology was used to optimize the ultrasonic extraction process of polysaccharides from Synsepalum dulcificum seeds. Based on the results of single factor experiments,response surface methodology was employed to evaluate the interactive effects and significance of three crucial variables including ratio of material to liquid,ultrasonic time,extraction temperature,mathematical model for the yield of polysaccharides from Synsepalum dulcificum seeds as functions was established.The optimum conditions were as follows: ratio of material to liquid 1 ∶ 43( g / m L),ultrasonic time 40 min,extraction temperature 50 ℃. Under the optimized conditions,the yield was up to 12.33%,which was in close to the predicted value of 12.36% by the model,indicated that the model was reasonable. The in vitro antioxidant assay results showed that Synsepalum dulcificum seed polysaccharides exhibited a dose- dependent scavenging activity against DPPH radical ABTS+radical,·OH radical and O-2·radical.Its hemi-inhibiting concentrations( IC50) were 0.31,0.41,0.24 and 0.19 mg /m L.But antioxidant activity of sample polysaccharides was weaker than those of vitamin C. This suggested that Synsepalum dulcificum seed had high antioxidant activity and the antioxidative activity was increased with polysaccharides concentration increasing.