Abstract: The bentonite solution of three initial turbidities was flocculated by chitosan of different molecular weight.A relationship curve between molecular weight and optimal concentration of chitosan was obtained and a relationship model between molecular weight and optimal concentration of chitosan developed by the fitting method.The coefficient between model simulation results and experimental data is respectively 0.990 2,0.986 4 and 0.985 2 under the three initial turbidities.The model was tested by chitosan flocculation experiments for raw water in actual water works,and the coefficient between experimental data and model simulation results was 0.981 8.