The invention discloses a preparation method of battery-grade lithium dihydrogen phosphate. The preparation method comprises the steps of uniformly mixing phosphoric acid and ethanol; slowly adding battery-grade lithium carbonate to generate double decomposition reaction; crystallizing and separating out lithium dihydrogen phosphate precipitate; filtering after the double decomposition reaction; remaining the filter residue; washing the filter residue and drying to obtain battery-grade lithium dihydrogen phosphate. According to the preparation method of battery-grade lithium dihydrogen phosphate, based on the characteristic of low solubility of lithium dihydrogen phosphate in ethanol, phosphoric acid and battery-grade lithium carbonate react in the presence of ethanol and then battery-grade lithium dihydrogen phosphate is extracted, crystallized and separated out. With the adoption of the preparation method, the problem of high energy consumption due to evaporation and concentration and cooling crystallizing in the traditional preparation method of battery-grade lithium dihydrogen phosphate can be avoided; therefore, the energy consumption is decreased, and the energy conservation and emission reduction can be achieved.