The invention relates to a preparation method of an antiscale agent and in particular relates to a preparation method of polyaspartic acid serving as an antiscale agent. The preparation method comprises the following steps: adding maleic acid and urea according to the molar ratio of 1:0.85 to 1:0.95 into a 250ml of three-neck flask, stirring, heating water bath to 92.5 to 97.5 DEG C and reacting for 3.25 to 3.75 hours to obtain maleic acid ammonium salt; adding the maleic acid ammonium salt and phosphoric acid into the 250ml three-neck flask according to the mass ratio of 1:0.5 to 1:0.65, adding 100ml of decanol, stirring, heating to 190 DEG C, reacting for 40 minutes, cooling to normal temperature, performing suction extraction, washing with ethanol and deionized water sequentially, and drying to obtain polysuccinimide; adding 5.0g of polysuccinimide into a 100ml of flask, adding 6.5 to 7.5ml of NaOH solution being 2mol/L and a certain amount of deionized water, heating water bath to 35 to 45 DEG C, hydrolyzing for 45 to 55 minutes, adjusting sodium polyaspartate solution to be neutral by using 10% of hydrochloric acid, adding ethanol, mixing and stirring, filtering and drying to obtain polyaspartic acid. The raw materials are easily available, the cost is low and the preparation method is simple.