The invention belongs to the field of material chemistry, and discloses a preparation method of nano iron phosphate, which comprises the following steps: (1) adding a ferric salt and a phosphate intowater to obtain a solution A; (2) adding a surfactant into the solution A to obtain a solution B; (3) stirring the solution B, carrying out ultrasonic treatment, and carrying out a microwave hydrothermal reaction in a microwave digestion instrument to obtain a solid C; and (4) drying and calcining the solid C to obtain the nano iron phosphate. A microwave hydrothermal method is adopted, the methodhas the advantages of being high in heating speed, uniform in heating, free of temperature gradient and the like, a conventional hydrothermal reaction needs 2-8 h, microwave hydrothermal reaction only needs 3-15 min, and the sample preparation time is greatly shortened. The iron phosphate prepared by the invention is nanoscale, has the particle size of 20-30nm, uniform particle size, no agglomeration phenomenon and high tap density, can be used as a precursor of battery-grade lithium iron phosphate, and has the advantages of simple preparation process, high production efficiency, high productpurity and good dispersion performance.