A preparation method for a cathode material of lithium manganese iron phosphate, comprising the following steps: (1) weighing a lithium source, a manganese source, a phosphorus source, and an iron source according to a stoichiometric ratio of lithium, to manganese, to iron, and to phosphorus in LiMnxFe1-xPO4, wherein 0.1 ≤ x ≤ 0.8; (2) mixing the lithium source, the manganese source, the phosphorus source, and the iron source with a reductant, performing wet grinding, and then drying to obtain a powder material, wherein the reductant comprises a reductant A which undergoes sublimation at 500-800 °C and has reducibility within this temperature range, and comprises or does not comprise a carbon source; and (3) heating the powder material to 400-500 °C in a protective atmosphere, maintaining it at this temperature for 0.5-2 h, then heating to 500-800 °C, and sintering for 5-12 h to obtain the cathode material of lithium manganese iron phosphate. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein the reductant A is at least one of arsenic sulfide, molybdenum sulfide, and sulfur. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 2, wherein the reductant A is arsenic sulfide. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 3, wherein an amount of substance of the carbon source is a, an amount of substance of the reductant A is b, and an amount of substance of the iron source is c, (a+3b): c= 1: (1-1.5); and a: b= (0-0.3): (0.3-0.5). The preparation method for the cathode material of lithium manganese iron phosphate according to claim 4, wherein a: b= (0.1-0.3): (0.3-0.5). The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; the manganese source is at least one of manganese phosphate, manganese oxalate, manganese carbonate, manganese chloride, manganese sulfate, and manganese nitrate; the phosphorus source is at least one of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; and the iron source is at least one of iron oxide, iron nitrate, iron chloride, iron iodide, iron bromide, iron sulfide, and iron hydroxide. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein, in step (2), a method for the wet grinding comprises: adding water, ethanol, or a mixture thereof to a mixture of the lithium source, the manganese source, the phosphorus source, the iron source, and the reductant to perform the wet grinding, and stopping the wet grinding when a particle size of powder is less than 5 μm; and the drying is performed under vacuum at 50-65 °C for 5-12 h. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein, in step (3), heating to 400-500 °C is performed at a rate of 5-10 °C/min, and heating to 500-800 °C is performed at a rate of 1-5 °C/min. The preparation method for the cathode material of lithium manganese iron phosphate according to claim 1, wherein, in step (3), after the sintering, a product is subjected to airflow crushing to obtain the cathode material of lithium manganese iron phosphate with a particle size of D10 ≥ 0.4 μm, D50 of 0.5-0.8 μm, and D90 ≤ 1 μm. A cathode material of lithium manganese iron phosphate prepared by the method for the cathode material of lithium manganese iron phosphate according to any one of claims 1-9.