It is of great importance to separate potassium salts from carbonate-based lakes, especially the low-[K+] but high-[Nal brine which has been discovered around the world. Here the phase diagram of the quaternary system Na+, K+/Cl-, CO32--H2O was experimentally studied at 298.15 K via the method of isothermal evaporation, and the results corrected the previous diagram reported by Blasdale (J. Am. Chem. Soc. 1923, 45, 2935-2946). The type of salts appeared and their solubility during the evaporation, and the density vs composition diagram and the pH values were plotted based on the experimental data. It was found that there were six invariant points, seven univariant curves, and five crystallization fields in the equilibrium phase diagram. To extract single-salt of potassium, certain areas in the Na+, K+//C1-, C032 --H2O diagram were chosen to be acidified by the compressed CO2, and potassium in the form of KHCO3 was precipitated from the mother solution. The compressed CO2 was able to avoid formation of NaKCO3.6H2O during evaporation of the original system Na+, K+//C1-, CO32--H2O. Based on experiment data, the region area for potassium salt extraction was determined, and this area was suitable for extracting K+ salts from brines with a [Na+/ICl ratio even lower than 0.44. This strategy opened a new way to recovery of potassium with a maximum yield of 59.88% from the carbonate-chloride -based brine.