The invention discloses a method for preparing monopotassium phosphate. The method comprises the following steps of: 1) preparing normal-temperature saturated solution of potassium sulphate; (2) agitating the saturated solution of potassium sulphate and slowly adding monocalcium phosphate, controlling the local temperature to be lower than 35 DEG C and the reaction time at 6 hours, so as to obtain stand-by solution; 3) carrying out filter pressing on the stand-by solution to obtain stand-by filtrate; 4) concentrating the filtrate by reduced pressure until the volume is 40-50% of volume of the filtrate, and carrying out suction filtration on the filtrate, so as to obtain filtrate; 5) cooling the filtrate to room temperature, wherein the separated crystal is monopotassium phosphate. By adopting the method for preparing the monopotassium phosphate, the by-product is calcium sulfate, and can be used as a soil conditioner. No pollution is generated in the production process; no waste gas or waste water is generated; and the production cost is low.