OBJECTIVE：To optim ize the water extraction-ethanol precipitation technology of polysaccharide from Litchi chinensis seed，and to evaluate its hypoglycemic activity in vitro . METHODS ：The content of polysaccharides was determined by phenol-sulfuric acid colorimetry ，and the extraction rate of polysaccharides was calculated. Single factor test and response surface methodology were used to optimize the water extraction technology with the ratio of material to liquid ，extraction times and extraction time as factors ，and the extraction rate of polysaccharide as index. Single factor test was used to screen the concentration volume fraction of water extract and ethanol precipitation Using acarbose as contro l，4-nitrophenol-α-D-glucopyranoside method was used to investigate in vitro inhibitory activity of polysaccharide from L. chinensis seed to α-glucosidase. RESULTS ：The optimal technology was the ratio of material to liquid 1∶19 （g/mL），decocting for 3 times，1 h for each time ，concentrating the water extract to 40％ of original volume ，and adding ethanol to 80％ volume fraction. After deproteinization by Sevage method ，the crude polysaccharide of L. chinensis seed was obtained. The results of 3 times of validation tests showed that ，extraction rates of polysaccharide were 7.61％，7.89％，7.99％，average extraction rate was 7.83％（RSD＝2.52％，n＝3）. The contents of polysaccharide were 55.57％，55.83％ and 56.66％，average content was 56.02％（RSD＝1.81％，n＝3）. The inhibitory activity of the polysaccharide from L. chinensis seed to α-glucosidase were increased as concentration ；its IC 50 was 0.056 mg/mL，which was lower than positive control acarbose （0.196 mg/mL）. CONCLUSIONS：The optimal water extraction-ethanol precipitation technology of polysaccharide from L. chinensis seed is stable and feasible. The polysaccharide from L. chinensis seed show significant in vitro inhibitory effect on α-glucosidase，which is better than that of acarbose.