Objective: In order to explore the extraction method and antioxidant activity of crude polysaccharide from edible fungus resources of reed. Methods: The characteristic Agrocybe praecox of reed edible fungus of Dongting Lake District was used as raw material, the extraction rate of polysaccharide was used as the index, and the extraction was carried out by ultrasonic-assisted hot water alcohol precipitation method. After taking the liquid-material ratio, ultrasonic extraction time, ultrasonic extraction power, hot water extraction temperature, hot water extraction time as the factors to finish the single factor experiments. Then the experiments was optimized by Box-Behnken center combined experimental design for the crude polysaccharide extraction process of Agrocybe praecox. The scavenging abilities of the crude polysaccharide on DPPH· free radical and ABTS⁺· free radical were also studied. Results: The results showed that the optimal extraction conditions were determined: the ratio of solid to liquid of 1∶50 (g/mL), ultrasonic extraction time of 20 min, eextraction power of 150 W, Hot water extraction temperature of 80 ℃, extraction time of 4 h. Under the conditions, the extraction rate of crude polysaccharide was 5.08%. The scavenging rates of DPPH· and ABTS⁺· were 55.05% and 58.47%, respectively. The semi-inhibitory concentrations of IC₅₀ were 1.03 and 0.28 mg/mL, respectively. Conclusion: Under the optimal process parameters of ultrasonic-assisted hot water extraction, the extraction rate of polysaccharide is high, and the crude polysaccharide has a certain antioxidant activity in vitro.