Response surface methodology (RSM) using a Box-Behnken design (BBD) was applied to optimize the extraction of Paxillus involutus polysaccharides. The optimum conditions included an extraction time of 3 h, extraction temperature of 79 °C and a ratio of liquid to raw material of 43.1 mL/g. Under the optimized conditions, the polysaccharides yield was 12.25%. Then, the polysaccharides were purified with DEAE-Cellulose 52 and Sephadex G-100 gel columns, and the fraction denoted as PIP2-1 with a molecular weight of 32 kDa was obtained. PIP2-1 was composed of mannose, glucose, galactose, fucose with the mole percentages of 2.8%, 62.2%, 25.4% and 9.6%. The PIP2-1 possessed typical Fourier Transform infrared spectroscopy (FTIR) characterization of polysaccharides. The methylation analysis showed that the PIP2-1 mainly included 1-linked-fuc, 1,3-linked-man, 1-linked-glc, 1,4-linked-glc, 1,6-linked-glc, 1-linked-gal, 1,6-linked-gal, 1,4,6-linked-gal and 1,2,6-gal glycosidic bonds. Furthermore, PIP2-1 showed significant antioxidant activity against hydroxyl radicals (OH), 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH), 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) and superoxide radicals. Finally, PIP2-1 significantly enhanced the release of TNF-α and IL-6 in RAW264.7 cells. The results indicated that PIP2-1 could be exploited as a natural antioxidant and immunomodulator for functional food and medical applications.