Undaria pinnatifida, which is mainly distributed in Liaoning, Shandong and Zhejiang province in China, is a kind of edible seaweed belongs to Phaeophyta. In the present study, a sulfated polysaccharide named as UPP was extracted by hot water from Undaria pinnatifida with a yield of 11.00 ± 0.25%. The molecular weight of UPP was 691.05 kDa, and the contents of carbonhydrate, uronic acid, sulfate and protein were 50.02 ± 1.14%, 14.98 ± 0.52%, 11.19 ± 0.40% and 2.37 ± 0.17%, respectively. UPP was composed by glucose, xylose, mannose, arabinose, fucose, galactose and rhamnose, while glucose and xylose contributed to a total molar percentage of 71.39%. UPP consisted of (1→2) or (1→4) glycosidic bond by 62.36% as well as (1→3) glycosidic bond by 35.78%. Fourier transform-infrared (FT-IR) spectra indicated that UPP have characteristic functional groups of polysaccharides. Three different methods were applied to investigate the antioxidant activity of UPP in vitro. The results showed that oxygen radical absorbance capacity(ORAC), ABTS scavenging capacity value and DPPH scavenging capacity value of UPP were 164.54 ± 4.85 μmol TE/g，64.93 ± 4.76 μmol Trolox/g and 46.80 ± 6.06 μmol Trolox/g. In addition, the immunomodulatory activity of UPP was examined by murine macrophage RAW264.7 cells model. The results indicated that UPP presented unsinificant cytotoxicity to RAW264.7 cells with the concentration of 50~1500 μg/mL. Meanwhile, RAW264.7 cells showed the strongest pinocytic capability (113.22 ± 2.75% to the control) when the concentration of UPP was 600μg/mL. Furthermore, UPP obviously promoted the production of NO, which ranked the highest when the concentration of UPP was 600μg/mL. The mRNA of iNOS increased significantly with the concentration of UPP increased from 100~800μg/mL. These results above suggested that UPP possessed excellent antioxidant ability and immunodulatory activity and could be applied to industrial production of healthy foods as an effective component.