Abstract: A method to prepare chemical manganese dioxide by directly oxidizing manganese hydroxide with air in the presence of catalyzer was investigated. The processes of the method included that manganese hydrolyzate was prepared by hydrolysis of manganese sulfate in ammonie medium, the hydrolyzate furthermore hydrolyzed to manganese hydroxide, and manganese hydroxide was oxidized to MnO2 at certain temperature with ir in the presence of catalyzer. The research results indicate that the oxidation rate of manganese hydroxide is slow and the obtained oxidization product contains MnO2 less than 66%. The catalyzer R can elevate the oxidization rate of manganese hydroxide, which is influenced by temperature and the particle size of manganese hydroxide. Elementary manganese dioxide with MnO2 more than 83% and an apparent density of 1.7-1.8 was yielded in the presence of the catalyzer R and by utilization of optimized reaction parameters.