The present invention relates to a method for preparing a lithium metal phosphor oxide, the method including: mixing an iron salt solution and a phosphate solution in a reactor; applying a shearing force to the mixed solution in the reactor during the mixing to form a suspension containing nano-sized iron phosphate precipitate particles; obtaining the nano-sized iron phosphate particles from the suspension; and mixing the iron phosphate with a lithium raw material and performing firing, and the lithium metal phosphor oxide according to the present invention has an Equation of LiMnFePO4. Herein, M is selected from the group consisting of Ni, Co, Mn, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, and Mg, and n is in a range of 0 to 1. According to the present invention, since calcination is performed at a temperature that is lower than that of another existing method, there is an effect of reducing a process cost, and the obtained lithium metal phosphor oxide prepared according to the method of the present invention has an olivine structure type.