Abstract: In order to optimize the technology for the water extraction of polysaccharides from Chlorella pyrenoidosa, orthogonal test considering four factors including the ratio of water to raw material, ultrasonic power, ultrasonic time and extraction times were conducted. Free proteins were removed by TCA method. The results showed that the optimum extraction conditions were obtained as follows: the ratio of water to raw material was 15, extracting temperature was 600W, extracting time was 6 min and extracting times was 2, which led to estimated values of maximal yield of polysaccharides of 5.918%. Conditions of free proteins removed by TCA method was pH4 and the recovery rate of purified polysaccharides was up to 57.84%, which provide an industrial reference for developing new processes of polysaccharides production from Chlorella pyrenoidosa