The invention discloses lithium manganate composite manganous-manganic oxide and an industrial preparation method thereof. The method comprises the following steps: crushing high-purity manganese metal powder till the particle size of 10-300mu m by using a dry method, grading the manganese metal powder by using grading equipment, putting into a reactor with pure water and an ammonium salt, introducing air for oxidation, stirring at a speed of 100-500r / minute, introducing air at a flow of 100-300m<3> / hour, reacting for 20-25 hours, adding dissoluble manganese salt, sufficiently stirring, further adding bicarbonate, continuously reacting for 3-5 hours till the reaction is completed, further washing, drying, and roasting, thereby obtaining a composite manganous-manganic oxide product. The composite manganous-manganic oxide prepared by using the method is prevented from lithium manganate lattice oxygen defects, product properties and consistence are improved, meanwhile the amount of blownair is reduced, and the energy consumption of a kiln is reduced.