The apparatus includes a shell, hollow rotary shaft, spiral blades and vent duct. Manganese carbonate is charged through the feed port and the oxygen bearing mixed gas preheated to the required reation temperature is fed through the hollow rotary shaft hole into the apparatus for reaction; it makes O2 in full contact with MnCO3, controls even temperature, gives stable reaction condition and thorough pyrolysis, and yields MnO2 of gamma and beta crystal structure with excellent discharging property. It has such advantages as simple structure, limited investment, high utility of O2 and heat, selectable size, easy operation, good product quality and low production cost.