The invention discloses a terpolymer scaling inhibitor. The terpolymer scaling inhibitor is prepared from the following raw material components by weight percent: 2.0 to 7.2 percent of itaconic acid, 1.6 to 5.8 percent of 2-acrylamide-2-methylpropanesulfonic acid, 1.8 to 2.0 percent of hydroxyethyl methylacrylate, 0.3 to 0.8 percent of ammonium persulfate, 0.3 to 0.8 percent of tertiary butanol and the balance of deionized water, and the sum of the weight percent of the components is 100 percent. The preparation method comprises the following steps: mixing the itaconic acid, the 2-acrylamide-2-methylpropanesulfonic acid, the hydroxyethyl methylacrylate and the deionized water to obtain a solution A; mixing the tertiary butanol and the deionized water to obtain a solution B; mixing the solution A and the solution B to obtain a solution C; mixing the ammonium persulfate and the deionized water to obtain a solution D; and heating the solution D to 80 to 95 DEG C, dropwise adding the solution C under a stirring condition, reacting at constant temperature, and cooling to obtain the terpolymer scaling inhibitor. The terpolymer scaling inhibitor has the characteristics of less consumption and low cost, and the preparation method is simple, fast and low in manufacturing cost.