A new nitrogen-free and phosphorus-free scale-inhibitor modified polyaspartic acid was synthesized in the water system. It contains hydroxyl, sulfonic acid groups and carboxyl groups. Explore the optimum synthetic condition as follows : the moles ratio of n (maleic acid): n (ammonia):n(citric acid): n-(2-amino-ethanesulfonic acid) is 1:2.8:0.12:0.075 ; reaction time is 3 hours; the thermal polycondensation temperature is 190℃; the polymerization time is 1 h.The product’s structure was characterized. A conclusion is drawn that scale inhibition rate of the agent was 82.9% when the dosage reaches on 24 mg／L .The new antiscalant is significantly better than the traditional productions.