Abstract: Objective: To study the microwave treatment-ultrasonic extraction of S.tonkinensis polysaccharose and its biological activity.Methods: The polysaccharide yield and the purity of polysaccharide as index, the orthogonal experiment was used to optimize the extraction conditions of polysaccharose from S.tonkinensis through microwave treatment-ultrasonic extraction, and the stability, antioxidant activity and nitrite scavenging activity were also studied.Results: The optimum technological conditions were as followed: desorption agent ratio of 1∶ 5 ( g/m L) , microwave time of 30 s, solid/liquid ratio of 1 ∶ 25 ( g/m L) , ultrasonic power of 140 W, extraction time of 20 min.Under the optimum conditions, the yield of polysaccharose was 3.27%, the purity of polysaccharide was 29.49%, the extraction effect was better than hot water extraction and ultrasonic extraction. The stability result showed that polysaccharose from S.tonkinensis was more stable under the conditions of temperatures of 40~70 ℃, Ca²⁺ or citric acid, but it was less stable under the conditions of higher than 70 ℃, H₂O₂, Na₂SO₃, VC, Na⁺, Al³⁺, Cu²⁺ or Fe³⁺.The antioxidant activity research showed that polysaccharose from S. tonkinensis had antioxidant activities. When the concentration was 1.96 mg/m L, the clearance rates of microwave treatment-ultrasonic extraction polysaccharides from S. tonkinensis to O-2·and ·OH were 71.16% and 78.14%, respectively. The results of nitrite scavenging indicated that S. tonkinensis polysaccharose had a good nitrite scavenging activity. When the extraction volume was 20 m L, the removal rate of microwave treatment-ultrasonic extraction polysaccharides was 82.94%, and the removal efficiency was equivalent to 0.32 mg VC. The O-2· and nitrite clearance activities of S. tonkinensis polysaccharides extracted by microwave treatment-ultrasonic were equivalent to the polysaccharides extracted by ultrasonic at the same concentration, and the ·OH clearance rates of polysaccharides extracted by microwave treatment-ultrasonic extraction was better than the polysaccharides extracted by ultrasonic. Conclusion: S.tonkinensis was abundant with polysaccharide, which had good antioxidant activity, nitrite scavenging activity. But the stability of S.tonkinensis polysaccharide was poor, which should be stored in cool and dark condition.