Preparation method of lithium iron phosphate

The invention discloses a preparation method of lithium iron phosphate. Ferroferric oxide reacts with phosphoric acid, then iron powder is added for reaction, and then filtering is conducted to obtaina first filtrate and first filter residues; the obtained first filtrate, ammonia water and hydrazine hydrate are added into a bottom solution, and then the mixture is filtered to obtain a second filtrate and second filter residues; the second filter residues are washed with hot water, dried, sieved, subject to iron removal and placed into a roasting furnace for calcination, and anhydrous ferrousphosphate is obtained; anhydrous ferrous phosphate, manganese dioxide, polyethylene glycol 4000 and lithium phosphate are added into pure water and stirred and mixed in a stirring mill, then spray drying is carried out on the ground material, the dried material is calcined in a sintering furnace under a nitrogen atmosphere, and the product is cooled to room temperature and discharged; the productis crushed, screened, subje