Caulerpa lentillifera is a kind of perennial large edible seaweed, which is also known as "sea grape" because of its upright stem globule glittering and translucent, similar in appearance to grapes. C. lentillifera is also one of the most suitable seaweeds for human consumption. It is rich in nutrients and contains a variety of amino acids, vitamins, polysaccharides and fatty acids needed by the human body. Among them, polysaccharide is the most important active component in C. lentillifera. According to research, the crude polysaccharide content of C. lentillifera accounts for 34.00%-64.97% of its dry weight, which is higher than that of most edible algae, such as Scagassum, Hizikia fusifarme and Undaria pinnatifida. At present, the main extraction methods for polysaccharides of C. lentillifera are water extraction, alkali extraction and ultrasonic-assisted extraction, and the extraction rate is 3.22%-11.30%, but there are still a large amount of polysaccharides that remains un-extracted. In order to develop a mild, efficient and low-cost extraction process of polysaccharides from C. lentillifera, and to explore the antioxidant activity of the polysaccharide, we screened five existing extraction methods of polysaccharides from C. lentillifera. Through single factor experiment and response surface experiment, the ratio of material to solution, extraction temperature, extraction time, amount of papain and extraction times were optimized, and the optimum process parameters were obtained. Polysaccharide concentration was determined by anthrone sulfate method in the experiment. In addition, we investigated the scavenging activity of the polysaccharide on DPPH and ABTS free radicals. The results showed that adding papain is the most efficient method to extract polysaccharides from C. lentillifera. The order of the factors affecting the extraction rate of polysaccharide from C. lentillifera is as follows: ratio of material to solution > extraction temperature > extraction time, and the ratio of material to solution showed an extremely significant influence on the extraction rate of the polysaccharide (P<0.001). The best extraction parameters after optimization were as follows: ratio of material to solution is 1∶45, extraction temperature 60 ℃, extraction time 3 h, extraction count 2 times, the addition amount of papain 2.0%. Under this condition, the extraction rate of polysaccharides can reach 41.24%±0.09%. In addition, polysaccharides from C. lentillifera had good scavenging activities on DPPH and ABTS free radicals, and their IC₅₀ values were 2.32 and 0.67 mg/mL, respectively. This study shows that the optimized papain enzymatic hydrolysis can improve the extraction rate of polysaccharides from C. lentillifera effectively, among which the ratio of material solution is the most important factor affecting this method. Besids, the polysaccharides of C. lentillifera have good antioxidant activity.