Abstract: The orthogonal experiment method was used to optimize the parameter of preparation of low molecular weight chitosan by hydrogen peroxide oxidation degradation combined with microwave and ultrasonic radiation degradation. The structure of low molecular weight chitosan was characterized by infrared spectroscopy and differential thermal analysis.The orthogonal experiment showed that the optimization parameters of preparation of low molecular weight chitosan were summarized as follows: the concentration of H2O2 was 8%,and the reaction time was 10 minutes combined with microwave and ultrasonic radiation degradation 3 minutes. The average molecular weight of product was 3.52 ku.Infrared spectrum analysis showed that the low molecular weight chitosan keeped the molecules skeleton structure and the β( 1 → 4) glycosidic bond was fractured partly. The degree of deacetylation increased in the course of degradation. Thermogravimetric analysis showed that thermal stability of low molecular weight water- soluble chitosan was lower than that of raw materials chitosan.