The invention discloses a device for preparing lithium dihydrogen phosphate, which can slow down the corrosion to a negative pressure evaporator. The device for preparing the lithium dihydrogen phosphate comprises the negative pressure evaporator and a heating circulation chamber, wherein a circulating pump is arranged between the heating circulation chamber and the negative pressure evaporator; the negative pressure evaporator, the circulating pump and the heating circulation chamber are connected together in sequence to form a circulation loop; and the heating circulation chamber is connected with a heating device. When the device works, solution of the lithium dihydrogen phosphate is circulated between the negative pressure evaporator and the heating circulation chamber so as to avoid an over-high temperature appearing on the inner wall of the negative pressure evaporator and greatly reduce the corrosion to the negative pressure evaporator; and simultaneously, in the circulation process, internal circulation spaces of the negative pressure evaporator and the heating circulation chamber can be used fully, so the device improves the heat exchange efficiency, is more favorable for the evaporation concentration of the solution of the lithium dihydrogen phosphate, can avoid product deposition caused after the concentration of the solution of the lithium dihydrogen phosphate, can make crystals uniform, and is suitable to be popularized and used in preparing various acidic chemical substances.