The invention discloses a method for preparing lithium manganese iron phosphate by a solid phase method. The preparation method comprises the following steps: weighing a certain amount of a manganese source and an iron source according to a molar ratio of 7:3, weighing a lithium source, a phosphorus source, a carbon source and a dopant according to a certain stoichiometric ratio, adding pure water, carrying out ball milling and sanding, controlling the sanding particle size D50 to be less than or equal to 300 nm, and carrying out spray drying to obtain brown precursor powder; and sintering the precursor under the protection of a nitrogen atmosphere, controlling the sintering temperature to be 600-700 DEG C, then performing crushing and screening, and removing iron to obtain the lithium manganese iron phosphate positive electrode material. The lithium manganese iron phosphate prepared by the method is simple in process and easy to control in process, compared with existing lithium iron phosphate and ternary materials, the lithium manganese iron phosphate is lower in cost and higher in voltage platform, and meanwhile, the obtained lithium manganese iron phosphate has good electrical performance and cycle performance.