Lithium iron phosphate material has become the preferred cathode material for lithium-ion power batteries due to its simple preparation process, long product cycle life, high temperature stability, low cost and high safety performance, and has become a hot spot for new energy research in various countries. At present, the world's leading lithium iron phosphate technology manufacturing companies, including A123, Valence and Phostech companies, China's main lithium iron phosphate battery companies, mainly led by BYD company. One or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, etc. as the source of phosphorus, carbon source as an additive, phosphorus source, iron source, lithium source of the substance ratio of 1:1: (1 ~ 1.05) for mixing, in a protective atmosphere for two sintering to produce lithium iron phosphate. The products prepared by this method can well balance high volumetric specific capacity and excellent high current discharge performance. Lithium-iron-phosphates were synthesized under hydrothermal conditions, and this method heated at least one lithium-containing compound, at least one iron-containing compound with 0 oxidation state of iron, and at least one phosphorus-containing compound with +5 valence oxidation state (e.g., NH4H2PO4) at 100-500 °C and autogenous pressure to produce particles or agglomerates with the general formula: Lia-bM1bFe1-cM2cPd-eM3eOx substances. This method is a simple process without any disturbing by-products and without calcination and purification steps.