The present invention relates to a method for preparing a lithium iron phosphate nanopowder, including the steps of (a) preparing a mixture solution by adding a lithium precursor, an iron precursor and a phosphorus precursor in a reaction solvent, and (b) putting the mixture solution into a reactor and heating to prepare the lithium iron phosphate nanopowder under pressure conditions of 10 to 100 bar, and a lithium iron phosphate nanopowder prepared by the method. When compared to a common hydrothermal synthesis method and a supercritical hydrothermal synthesis method, a reaction may be performed under a relatively lower pressure. When compared to a common glycothermal synthesis method, a lithium iron phosphate nanopowder having effectively controlled particle size and particle size distribution may be easily prepared.