Abstract: Objective: To improve the polysaccharide content and biological activities of the fermented Chinese yam polysaccharides (FCYP) by fermentation with Saccharomyces boulardii. Methods: The contents of polysaccharides, total protein and polyphenols of FCYP and unfermented Chinese yam polysaccharides (UCYP) were determined by phenol-sulfuric acid method, coomassie bright blue method and folin-ciocalteu method, respectively. The monosaccharide composition and physicochemical properties of FCYP and UCYP were determined by ultraviolet spectrum analysis, high performance liquid chromatography and Fourier transform infrared spectroscopy. Then the antioxidant activity and immunomodulatory activity were compared and analyzed. Results: Compared with UCYP (383.03±12.57 mg/g), the polysaccharide content in FCYP was significantly increased (480.71±5.93 mg/g, P<0.001). The Fourier transform infrared spectroscopy analysis showed that FCYP had typical polysaccharide functional groups (O-H and C-O) at 3279 and 1636 cm⁻¹, respectively. The monosaccharide composition of FCYP was mainly galactose and mannose, with the mass percentage of 48.70% and 16.19%, respectively. In addition, the results of antioxidant activity in vitro showed that FCYP had the best DPPH free radical scavenging activity in the concentration range of 1.0~5.0 mg/mL. And the reducing power of FCYP at 5.0 mg/mL was significantly higher than that of UCYP (P<0.05). Finally, the results of immunomodulatory activity showed that FCYP showed good enhanced cell proliferation activity and promoted NO production in RAW264.7 cells. Conclusion: It was an effective method to improve the polysaccharide content and biological activity of Chinese yam by fermentation with Saccharomyces boulardii. This study would provide a basis for the development of natural antioxidant products of fermented polysaccharides of Chinese yam.