According to action mechanism of water-reducer，the sodium salt of the terpolymer (PMMANa) was synthesized with maleic anhydride and diisobutyl maleate and acrylic acid as raw materials. The effects of synthesis conditions to water-reducing performance of the polymer were studied, the optimum synthesis conditions with orthogonal experiment were confirmed: the matching ester:anhydride:acid was 3∶4∶4, the dosage of initiator was 2%,the total amount of the monomers was 20%,the temperature was 70 ℃,the dosage of chain transfer agent was 3.0%.The water-reducing rate of the PMMANa synthesized at the optimum synthesis conditions was 35.19%, the water-reducing rate was higher than PAANa.