The scaling ions (Ca2+, Mg2+ and CO32–) in raw brine are easy to form calcium carbonate and calcium sulfate deposit in the process of vacuum evaporation. Because of their poor thermal conductivity and their good adherence to the walls, these mineral compounds decrease the heat transfer ratio and even shorten equipment life by corrosion. The brine purification method of NaOH–flue gas can utilize the flue gas from a power plant and low concentration waste alkali from chlor-alkali enterprises, which has absolute advantages and broad application prospects in energy conservation and environmental protection. In order to analyze the behavior of scaling ions in the process of brine purification by NaOH–flue gas method, the experiment of brine purification by CO2 instead of flue gas was carried out. The effects of CO2 feeding time and pH value on the behaviors of Ca2+, Mg2+ and CO32– in brine were investigated. The behavior of scaling ions and the mineralogical characteristics of precipitates were studied by thermodynamic calculation (Factsage), XRD and SEM–EDS. The thermodynamic and experimental results showed that when pH=11.5 and CO2 introduction time was 60 min, Ca2+ in raw brine was reduced to 6.0~8.0 mg/L, and Mg2+ concentration was reduced to 0.65 mg/L when the time was extended to 80 min. There was a linear relationship between the concentration of CO32– and CO2 entry time. Besides, the dissolution amount and ratio of CO2 in brine were the largest, and the equilibrium constant (K) was the largest at the pH=11.5. Then, the results of XRD and SEM showed that the precipitated vaterite type of calcium carbonate crystals was obtained when the pH of the process was controlled at 9.5. When the pH value increased to 10, the crystal form of calcium carbonate transformed from vaterite to aragonite crystals, and its morphology changed from a regular spherical shape to an oval shape. When the pH was 11 or 11.5, the precipitation was a eutectic of calcium and magnesium, and the crystal form of calcium carbonate was converted from vaterite to calcite crystals, which was more thermodynamically stable.