Optimum environment-friendly enzyme method for preparation of chitosan with high degree of deacetylation was established. Using freshwater crayfish shell as raw material,the chitosan was prepared by ultrasonic assisted CDA enzyme method after extraction of chitin by EDTA method. Then chitosan preparation technology combined with ultrasonic-assisted CDA enzymatic response was optimized by response surface method based on preliminary experiment. The results showed that the optimum enzymatic preparation condition for chitosan with high degree of deacetylation was as follows: the ultrasonic time 60 min,ultrasonic power 476 w,enzyme dosage 9. 45%,temperature50 ℃,hydrolysis time 3. 5h. Under this condition,the degree of deacetylation of chitosan was as high as 91. 31%,viscosity was 95 m Pa·s,and relative yield was 81. 87%. Compared with the ultrasonic method alone and the traditional alkali method,the above preparing process had remarkable advantages such as without environmental pollution,high degree of deacetylation,stability and so on,which would not only lay the foundation for the environmentally friendly industrialization production of chitosan,but also conform to the economic development trend in future.