Abstract: We have compared the degradation effects of ultrasonic method, hydrogen peroxide method and ultrasound-assisted hydrogen peroxide method on Porphyra haitanensis polysaccharide, and finally optimized ultrasonic assisted hydrogen peroxide degradation of polysaccharides by single factor and orthogonal test, using DPPH scavenging rate as the index. Meanwhile, we compared the antioxidant activity of polysaccharides and analyzed its correlation with molecular mass, structural characteristics and physicochemical properties. The results show that the IC₅₀ (Half maximal inhibitory concentration) of DPPH scavenging reduced from 9.37 mg·mL⁻¹ to 1.71 mg·mL⁻¹ under the conditions of 10% hydrogen peroxide, 65 ℃ and 2.5 h. Gel permeation chromatography shows that the molecular mass of polysaccharide decreased from more than 670 kD to 235 835 D before and after degradation. FT-IR shows that the absorption peaks of the polysaccharide did not change. The monosaccharide composition was almost the same with PMP column derivatization according to HPLC analysis, while the content of uronic acid decreased. Moreover, the Sulfate-group content increased but the 3,6-endogalactose content decreased obviously. Therefore, the ultrasonic assisted hydrogen peroxide method can degrade P. haitanensis polysaccharides and improve the antioxidant activity. The results provide references for the preparation of low molecular mass polysaccharides and analysis of structure-activity relationship.