Abstract The structural characterization and bioactivities of a polysaccharide from the stalk residue of Pleurotus eryngii were investigated. Firstly the stalk residue of P. eryngii was collected from the top portion of spent mushroom substrate and processed to yield water-soluble polysaccharide. Subsequently this crude polysaccharide was purified by DEAE Sepharose CL-6B ion exchange chromatography and Sepharose CL-6B size-exclusion chromatography. Then its structural features were investigated by gas chromatography (GC), gel permeation chromatography (GPC), methylation analysis, and Fourier Transform Infrared Spectrum (FT-IR). The results showed that it was heteropolysaccharide and mainly composed of glucose (82.4%). The backbone of P-2a mainly consisted of 1, 3-linked (42.7%) and 1, 6-linked (35.5%) glucose residues. Besides, in vitro antioxidant assay showed that P-2a exerted a high scavenging effects on hydroxyl radical and in vitro antitumor assay showed it had a dose-dependent antiproliferative effect against human gastric MGC-803 cancer cells & human epithelial Hela cancer cells. The findings of this study suggested that the polysaccharide extracted from the stalk residue has the similar structure and bioactivities as that from fruit-body of the mushroom. It could be potentially used as a natural source for the development of health-care food.