The invention relates to battery grade iron and manganese phosphate and a preparation method thereof. The chemical composition of the battery grade iron and manganese phosphate is MnxFe(1-x)PO4, wherein x is greater than or equal to 0.1 and smaller than or equal to 0.9; the preparation method comprises the steps of adopting a hydrothermal oxidation-coprecipitation technology, putting a soluble phosphorus source, iron source and manganese source solution into a reaction kettle according to the chemical composition MnxFe(1-x)PO4 of the iron and manganese phosphate, adding a surfactant and nitric acid, controlling the system pH value to be within 1 to 4, stirring for 2 to 48 h at 100 to 250 DEG C, so as to perform hydrothermal reaction, obtaining turbid liquid containing MnxFe(1-x)PO4.yH2O precipitate, naturally cooling the turbid liquid to room temperature, filtering, washing and drying, obtaining MnxFe(1-x)PO4.yH2O, performing high-temperature roasting at 250 to 700 DEG C, and then obtaining the battery grade iron and manganese phosphate not containing crystal water. The method is simple and practicable, and is easy to realize scale production, the prepared iron and manganese phosphate has the advantages that size distribution is uniform, the purity is high, and iron and manganese elements realize atomic-scale uniform distribution, and the battery grade iron and manganese phosphate a belongs to the optimal precursor for preparing lithium ferric manganese phosphate battery materials.