Chitooligosaecharide has a lot of special physiological and chemical functions.To obtain high quality chitooligosaccharide,this paper discussed the preparing technics of the chitooligosaccharide using the endo-chitosanase.The factor to effect the molecular weight of the hydrolysate was studied.The structure of the chitooligosaccharides of hydrolytic chitosan was determined by analysis of IR spectroscopy,and the polymerization degree was measured by HPLC.Experiment result showed that deacetylation affects the molecular weight degree and distribution of the polym erizafion degree.