To moniter the operation condition of circulating water system and realize real-time prediction of the pH variation of circulating cooling water, a mathematic model of pH variation tendency is set up and the mass transfer coefficientof gaseous CO2 is calculated. Then the validity of the model is verified by quasi-dynamic simulation experiment. Results illustrate that circulating water can achieve the gas-liquid equilibrium state after it fully contacts with gaseous CO2 in atmosphere, and the maximum error limit between the theoretical and the measured pH value is only 0.04. The pH variation tendency of quasi-dynamic system is in line with the predicting mathematic model and the A·KG is a constant when temperature(T）and spray flow(Q）are fixed. 10 5·A·KG=0.023 6Q－0.02 gives the relationship between A·KG and Q, when T is 40 ℃.