Cationic starch is a kind of sizing agent which have good prospect of application, but its high viscosity limits its use. In this work low substitution degree cationic starch (DS=0.03) was hydrolyzed with HCL to reduce the viscosity. The acidolysis factors on cationic starch viscosity such as reaction time, reaction temperature and concentration of HCL has been studied. The results revealed that the optimum reaction condition was 1.83% HCL for dry cationic starch, under 55°C for 5 hours, the product viscosity was 11.5mpa.s. By choosing different process parameters can get products of series viscosities.