One kind of papain was used to depolymerize chitosan in this paper. Four factors of reactions were studied including concentration of enzyme, pH, temperature and time. The result indicated that temperature influenced the depolymerization of chitosan efficiently, while time had relatively little effect on it with pH from 3.0 to 6.0, temperature from 30℃ to 60℃, and time from 2 to 8 hours. The data revealed that the enzymatic process did not obey a simple kinetic model; chitosan with average molecular weights in the range (5-10)×105 could be easily depolymerized to highly polydisperse chitosans under conditions such as pH 4.0, 40℃, 8 hrs, and papain 1.5 mg/mL.