In order to determine the change rule of polysaccharides content in fresh rice grains at different mature stages, the polysaccharides were extracted from fresh rice grains at different mature stages by ultrasonic-microwave synergistic extraction method. The composition of polysaccharides was analyzed by HPLC, and the antioxidant activity of polysaccharides was determined by antioxidant test in vitro. The results showed that the content of polysaccharides in fresh rice was the highest at the stage of late milk, up to 46.89 mg/g. Mannose, rhamnose, glucuronic acid, galacturonic acid, glucose, galactose, xylose, arabinose and fucose were the main components of polysaccharides at this stage, while mannose and fucose were not detected in polysaccharides of fresh rice at full ripening stage. The scavenging effects of polysaccharides on ABTS⁺, DPPH free radical, hydroxyl free radical and superoxide anion in fresh rice grains at later milk stage were 86.18%, 56.81%, 75.62% and 41.69% respectively, which were higher than those at other stages. This study has practical significance for the comprehensive utilization of fresh rice, and can provide reference for the development and utilization of fresh rice in functional food.