Background: Suillus granulatus is an edible and medicinal fungus in China. S. granulatus polysaccharide (SGP) was considered as the main bioactivity compounds in S. granulatus. Therefore, the extraction of SGP and their antioxidant activities were studied in this work. Materials and Methods: Fruiting bodies of S. granulatus were purchased from a local market (Fushun, China). Response surface methodology was adopted to optimize the extraction conditions of SGP. The antioxidant and immunological activities in vitro were also assayed. Results: The extraction of SGP was optimized by a Box–Behnken design. The optimal conditions for the extraction of polysaccharides were as follows: Pre-extraction time, 2 h; extraction temperature, 94°C; ratio of water to raw material, 25; and extraction frequency, 2. Under these conditions, the experimental yield of polysaccharides was 5.38% ±0.15%, which agreed with the predicted yield. The antioxidant assay in vitro showed that SGPs had relatively high scavenging ability for hydroxyl radicals and higher scavenging ability for 1,1-diphenyl-2-picrylhydrazyl radical. However, the scavenging ability of SGPs for superoxide anion radical and reducing power was relatively low. The polysaccharides also significantly increased splenocyte proliferation in vitro. Conclusion: SGP possessed good antioxidant and immunological activities in vitro and explored as a novel natural antioxidant or functional food.