The inhibition efficiency of polycarboxylic acid will decrease due to the weak acidity of carboxyl group in high Ca 2+ geothermal circulating water. To solve this problem, we synthesized a novel carboxyl terpolymer (MA-AEO-7/SMAS/MA) with adipate polyoxyethylene ether maleate monoester (MA-AEO-7), sodium methacrylate sulfonate (SMAS) and maleic acid (MA). Its inhibition efficiency and mechanism were investigated at high Ca 2+ . The results indicated that the inhibition efficiency of MA-AEO-7/SMAS/MA was 96.84% for CaCO 3 at the monomeric ratio of 0.5:1:1 and a dosage of 12 mg L À1 . The X-ray diffraction and scanning electron microscope results of CaCO 3 scale showed that the presence of MA-AEO-7/SMAS/MA converted CaCO 3 calcite to vaterite with smaller particle size. The molecular dynamics and density functional theory calculations indicated that the increased solubility of MA-AEO-7/SMAS/MA and the carboxylic acidity in MA-AEO-7/SMAS/MA enhanced the Ca 2+ chelating properties. The electrostatic repulsion between the sulfonic acid groups made MA-AEO-7/SMAS/MA exhibit good dispersion properties. These findings explained why the inhibition efficiency of MA-AEO-7/SMAS/MA remained above 80% at Ca 2+ concentration of 1500 mg L À1 , exceeding the 30%-50% of the three commercial scale inhibitors.This study provides practical insight for the development of scale inhibitors suitable for high Ca 2+ geothermal circulating water.