In this study， microwave-assisted extraction was used for the isolation of Phragmites rhizoma polysaccharide， its structure was characterized and its immunosuppressive effects in vitro were evaluated. The response surface method was used to optimize the extraction of polysaccharide. The optimal separation conditions of Phragmites rhizoma polysaccharide were microwave power 572 W， microwave time 22 min， liquid-solid ratio 15 mL/g， and the yield of Phragmites rhizoma polysaccharide was 1.61%. The monosaccharide composition of Phragmites rhizomapolysaccharide was arabinose， galactose and glucose， with a molar ratio of 3.03∶0.52∶6.46. Infrared analysis showed that Phragmites rhizoma polysaccharide may contain uronic acid and sulfuric acid groups. Phragmites rhizomapolysaccharide solution is colorless and transparent liquid. Scanning electron microscopy showed that Phragmites rhizoma polysaccharide was smooth and compact， with irregular sheet structure and uneven thickness. Cell analysis showed that 25-300 µg/mLPhragmites rhizoma polysaccharide could significantly improve the cell activity of macrophages and there was no significant difference between the polysaccharide groups and the blank group. 5-200 µg/mLPhragmites rhizoma polysaccharide significantly inhibited nitric oxide production in lipopolysaccharide-stimulated macrophages. 100 µg/mL Phragmites rhizomapolysaccharide significantly inhibited the production of inflammatory cytokines tumor necrosis factor α and interleukin 6 in macrophages stimulated by lipopolysaccharide. Therefore， Phragmites rhizoma polysaccharide may have good immunosuppressive effects.