Low hardness and low alkalinity circulating water (calcium ion is greater than 20mg/L, alkalinity is greater than 20mg/L) due to its strong corrosion and weak impact resistance, it has extremely high requirements for the performance of corrosion and scale inhibitors. According to the laboratory work in previous years, for low hardness and low alkalinity circulating water systems, low phosphorus or medium phosphorus formulas have relatively better corrosion inhibition performance, but for phosphorus-free formulas, it is more difficult . The normal operation is to add calcium chloride or sodium bicarbonate to the circulating water to increase its hardness and alkalinity, so as to achieve the standard of corrosion and scale inhibitors. The phosphorus-free formulation developed for the low hardness and low alkalinity circulating water system needs to overcome the above problems and find a new way to solve the problem. After testing, it was finally confirmed that the two-component alkaline formula, namely QD-685A1 and QD-685A2, were used. When the calcium ion is greater than 20mg/L and the alkalinity is greater than 20mg/L, QD-685A1 is dosed with 25~35mg/L, and QD-685A2 is dosed with 15~20mg/L. The corrosion rate is up to the standard. To simulate the circulating water quality conditions of Guangzhou Petrochemical, QD-685A1 was added with 25-30mg/L, QD-685A2 was added with 15-20mg/L, and the corrosion rate of the hanging piece was about 0.02mm/a.