The seed of Synespalum dulcificum was untrasonified in the acetone-(NH₄)₂SO₄ aqueous two-phase syetem to extract total polyphenils and different conditions for extraction of polyphenols antioxidant activity was investigated. Acetone was chosen as the best extraction solvent for this research. The acetone concentration, amount of the (NH₄)₂SO₄, the ultrasonic time，extraction temperature, ratio of water to solid were discussed by the single factor experiment. The optimal extraction conditions were found to be: the concentration of acetone solution 50%, ultrasonic time 100 min and extraction temperature 60 ℃，ratio of water to solid 20∶1(V∶m). The maximum extraction yield of total polyphenils was 11.54% in the optimum conditions. Under the optimum conditions, through duplicated experiment the yield of polyphenols was 11.56%, it indicated that the extraction rate of polyphenols was extremely stable. The purity of polyphenols extracted with this system is 87.85%, superior to only use ultrasonic, preliminary purification can be realized. The results of the experiment on different conditions for extraction of polyphenols antioxidant activity showed that it has strong ability of removing DPPH free radical and reducing power and to the polyphenol content. The maximum DPPH scavenging rate by polyphenols was up to 62%, higher then that by ascorbic acid.