A new supermolecular assembly crystal, [C 6 H 8 N 2 ] 6 H 3 [PW 12 O 40 ]·2H 2 O (DMB–PWA), was synthesized with phosphotungstic acid (PWA) and 1,2-diaminobenzene (DMB) under hydrothermal conditions and was characterized by Fourier-transform infrared spectra (FTIR) and single-crystal X-ray diffraction analysis. DMB–PWA could effectively catalyze oxidative degradation of chitosan with H 2 O 2 in the heterogeneous phase. The optimum degradation conditions were determined by orthogonal tests as follows: amount of chitosan 1.00g, 30% (wt %); H 2 O 2 , 3.0mL; dosage of catalyst, 0.06g; reaction temperature, 85°C; and reaction time, 30min. The water-soluble chitosan with a viscosity-average molecular weight (M v ) of 4900 was obtained under the optimum degradation conditions and was characterized by FTIR, ultraviolet–visible diffuse reflection spectra (UV–vis DRS), and X-ray powder diffraction analysis.