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 1 to 10 bar, and a lithium iron phosphate nanopowder prepared by the method. When compared to a common hydrothermal synthesis method, a supercritical hydrothermal synthesis method and a glycothermal synthesis method, a reaction may be performed under a relatively lower pressure. Thus, a high temperature/high pressure reactor is not necessary and process safety and economic feasibility may be secured. In addition, a lithium iron phosphate nanopowder having uniform particle size and effectively controlled particle size distribution may be easily prepared.