In this study, crude polysaccharides from Porphyra haitanensis (CPP) were obtained by water extraction and alcohol precipitation. The CPP were separated and purified by DEAE-cellulose column chromatography to give five components (PP1, PP2, PP3, PP4, and PP5) whose physicochemical properties were characterized. The sulfate-group content (20.1%) of PP3 was higher than those of the other four components. PP3 was treated with H2O2-VC, and then dialyzed to obtain degradation products PP3-1 (102 kDa), PP3-2 (72 kDa), PP3-3 (39 kDa), and PP3-4 (20 kDa). The antioxidant activities of the degradation products and PP3 were assessed, which revealed that PP3-4 was significantly more active than the other four components. PP3-4 was purified by Sephadex G-100 to give PP3-4a. The molar ratio of galactose to 3,6-anhydro-alpha-L-galactose in PP3-4a was 10:1. The structure of PP3-4a was characterized by FT-IR and 1D/2D NMR spectroscopies. The results reveal that the structure of PP3-4a contains two -> 3) beta-D-galactose (1 -> 4) 3,6-anhydro-alpha-L-galactose (1 ->, and -> 3) beta-D-ga- lactose (1 -> 4) alpha-L-galactose-6-S (1 -> repeating structural units, with the latter structural units dominating and partially substituted with methoxy groups at some C-6 positions of the beta-D-galactose and C-2 positions of 3,6-anhydro-alpha-L-galactose. The anomeric glycosidic bonds include both a and beta types.