The invention relates to the technical field of cathode materials for lithium ion batteries, in particular to a preparation method of an alumina-coated manganese-doped lithium iron phosphate cathode material, which comprises the following steps in turn: (1) ball milling a phosphorus source, an iron source, a lithium source carbon source and a reducing agent to obtain a manganese-doped lithium ironphosphate precursor; 2, adding a manganese source into a manganese-doped lithium iron phosphate precursor solution, successively dropping ammonia water, hydrothermal reacting, drying, calcining, andcooling to obtain that manganese-doped lithium iron phosphate material; 3) uniformly stirring that manganese doped lithium iron phosphate material and the organic solvent, adding aluminum salt solution in the stirring, and successively stirring, pumping, filter, drying, roasting, cooling to obtain an alumina coated manganese doped lithium iron phosphate cathode material; The alumina-coated manganese-doped lithium iron phosphate cathode material prepared by the invention can effectively improve the conductivity and the charge-discharge capacity, increase the electric contact area, and further improve the cycle performance of the lithium ion battery.