The invention discloses a method for preparing lithium ion phosphate, the cathode material of a lithium ion battery. The method is characterized in that the method comprises the steps of respectively taking lithium dihydrogen phosphate and soluble ferrous salt to be heated into be in molten state in non-oxidizing atmosphere, uniformly mixing the molten state raw material, and keeping the temperature at 400 to 550 DEG C for 0.5 to 5 h to obtain lithium ion phosphate. Compared with the prior art, the method has the advantages that the process route is simple, the period is short, the energy consumption is low, the safety is high, and the method is very suitable for mass production.