Based on dynamic modeling experiments in site, a comb i ned inhibitor mainly composed of molybdate/phosphonic acid/zinc cation/BTA has b een s tudied for soft circulating water in Maanshan Iron & Steel Company. Under the condition of total hardness of circulating water 10～20 mg/L,concentra t ion of MoO₄^2- 13～20 mg/L,phosphonic acid 15 mg/L,Zn²⁺4 mg/L,a res ult shows that the average corrosion rate of test tubes is 0.003 6 mm/a,avera ge fouling adherence rate is 7.0 mcm,while limiting fouling heating transfer res istance is nearly equal to zero. All the chemicals of the combined inhibitor is q uite stable in circulating water. The paper also discusses the operation of pre coating and dosage. The study shows that the molybdate combined inhibitor is ap plicable for the quality stability of soft water closed circulation system as well as low hardness co oling water system.