To improve the biological activity of polysaccharides from the fruit of Physalis alkekengi L., crude polysaccharides (PPSs) were isolated via water extraction and alcohol precipitation. The crude PPSs were deproteinized using the Sevage method, before being decolorized and purified with D101 macroporous resin. A homogeneous polysaccharide fraction, designated PPS-2A, was obtained through chromatographic purification using a DEAE-52 cellulose column and a Sephadex LH-20 gel column. Highperformance gel permeation chromatography revealed that PPS-2A had a molecular weight of 4.74 kDa. Its monosaccharide composition consisted of mannose, rhamnose, galacturonic acid, galactose, xylose, and arabinose at a molar ratio of 0.343 3: 0.832 3: 1.8505: 0.778 4: 0.744 2: 0.601 5. Using the nitric acid-sodium selenite method, PPS-2A was successfully selenylated to yield the selenylated polysaccharide Se-PPS-2A (yield: 60.13%, selenium content: 8.0 mg•g-1). The in vitro antioxidant activities of both PPS-2A and Se-PPS-2A were evaluated by assessing their reducing power and assays for DPPH, ABTS+, hydroxyl (•OH), and superoxide anion (O2-) radical scavenging. The results demonstrated that selenylation modification significantly enhanced antioxidant activity. Specifically, the DPPH scavenging rate increased from 80.22% to 86.38%, the ABTS+ scavenging rate increased from 75.42% to 81.22%, the maximum •OH scavenging rate increased from 41.06% to 52.40%, and the O2-• scavenging rate increased from 35.27% to 43.24%. The ferric reducing antioxidant power also increased from 0.78 to 0.84 mmol Fe2+/mg. In conclusion, selenylation modification significantly enhances the in vitro antioxidant activity of PPS-2A. These findings provide a theoretical foundation for the efficient utilization of Physalis alkekengi L. fruit resources.