Abstract: Abstract: Figs have traditionally been used as medicinal remedies for cancer and popular fruit of excellent source of nutrients. Polysaccharide is an important functional compound of fig and exhibits high anti-oxidant capacity. In order to promote the industrial development of fig and find out the effect of fig polysaccharide molecule modification methods on its activity, the objectives of this paper were 1) determine optimal conditions for water extraction and alcohol precipitation method by use of response surface design method; and 2) the antioxidant activity and molecular characterization of the extracted fig polysaccharides after treated with ultrasound. The traditional extraction method with water extraction and alcohol precipitation for fig were used in this study, and the extraction technology was optimization by response surface design method. And the molecular structure of polysaccharide was modified by ultrasonic wave. The optimal extraction conditions were: extraction time 21 min, extraction temperature 90℃, and the ratio of water to raw material 49:1. Under these conditions, the polysaccharide extraction efficiency was 3.03% for the first extracting, and 3.86%, and 94.62% for the following two extractions, which were all polysaccharide that can be extracted. Then the fig polysaccharides were modified by ultrasound under the condition of 600 W of ultrasonic power for 90 min with 5:2 (s:s) of ultrasonic on/off time ratio. The Fourier transform infrared spectroscopy analysis showed the absorption peak of 3 133 and 3 017 cm-1 was enhanced after the fig polysaccharides treated by ultrasound, which meant that a large number of C-O-C and C-O-H bonds were interrupted. The number average molecular weight and the weight average molecular weight of the polysaccharides were also decreased from 536 800, 1 061 000 Da down to 46 410, 93 870 Da determined by size exclusion chromatography with multi-angle light scattering. The ultrasound modified fig polysaccharides were graded into three groups PU-40, PU-60, PU-80 by 40%, 60% and 80% ethanol respectively. It was found that the PU-80 exhibited the highest antioxidant activity. The fig polysaccharides of PU-80 were further purification by Sephadex G-150 gel chromatography. Gel chromatography elution curves were asymmetry and trailing, which showed its low purity. The eluent solvent from tubes number 10 to number 20 were collected because of their largest peak area. After further condensed and freeze-dried, the best antioxidant component of PU-80 was obtained and labeled as PU-80-1. The number average molecular and the weight average molecular weight of PU-80-1 was about 58 810 and 157 300 Da determined by size exclusion chromatography with multi-angle light scattering. UV scanning analysis of PU-80-1 showed that there was a single absorption peak around 205 nm, which is the typical UV polysaccharide characteristic absorption peak. There were no obvious absorption at 260 and 280 nm, which showed that PU80-1 may not contain nucleic acids, polypeptides and other impurities. So it can be assumed that the substance was carbohydrate compounds. Infrared spectra analysis of PU-80-1 showed that it appeared strong absorption peak for the O-H stretching at 3 419 cm-1, and the weak absorption peaks at the 2 921 cm-1 for the C-H stretching vibration, and not too sharp absorption peak at 1 428 cm-1 for the C-H variable angle vibration. All of these illustrated existence of polysaccharide compounds because the absorption peak between 800-1 200 cm-1 basically belonged to C-O-C and C-O-H bond vibration. Gas chromatography analysis showed that the PU-80-1 was composed of monosaccharide of D-gl, L-rhamnose, D-glucose and D-galactose at ratio of 1.63:0.88:1.