Fuzhuan brick tea (FBT), one of the unique dark teas, has various health-promoting functions. In the present study, one polysaccharide fraction, namely FBTPS-2-1, was extracted and purified from FBT, and its structure and potential immunostimulatory activity were investigated. The results showed that FBTPS-2-1,one of typical heteropolysaccharides, was mainly composed of Gal, Ara, and Glc with little molar content of Man, Rha, GalA, and GlcA in molar ratio of 46.59:22.13:13.57:8.20:6.02:2.12:1.38 and molecular weight of 748 kDa. The backbone of FBTPS-2-1 contained →4)-β-d-Gal-(1→4)-β-d-Gal-(1→, →4)-β-d-Gal-(1→4)-α-d-Glc-(1→, →4)-α-d-Glc-(1→4)-α-d-Glc-(1→, →4)-α-d-Glc-(1→4)-β-d-Gal-(1→, →3)-β-d-Gal-(1→4)-β-d-Gal-(1→, →3,6)-β-d-Gal-(1→3)-β-d-Gal-(1→ and →3,6)-β-d-Gal-(1→3,6)-β-d-Gal-(1→. The linkages of branches in FBTPS-2-1 were mainly composed of α-l-Ara-(1→3,6)-β-d-Gal-(1→, →5)-α-l-Ara-(1→3,6)-β-d-Gal-(1→, →6)-β-d-Gal-(1→3,6)-β-d-Gal-(1→, α-l-Ara-(1→3,5)-α-l-Ara-(1→, →3,5)-α-l-Ara-(1→5)-α-l-Ara-(1→, α-d-Gal-(1→3,5)-α-l-Ara-(1→ and →5)-α-l-Ara-(1→6)-β-d-Gal-(1→. Furthermore, FBTPS-2-1 could increase the phagocytosis of macrophages and promote the secretion of NO and a variety of inflammatory cytokines, including TNF-α, IL-1β, and IL-6, indicating noticeable immune enhancement activity. Thus, FBTPS-2-1 could serve as a potentially functional food to improve human health by modulating the host immunoreaction.