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To fully understand the metallogenic regularity and the geochemical equilibrium rules of brine migration and transformation among potassium, barium and strontium in gas field water, the isothermal dissolution equilibrium method was used to study the phase equilibria of quaternary system KCl + BaCl2 + SrCl2 + H2O and its two ternary systems BaCl2 + SrCl2 + H2O and KCl + BaCl2 + H2O at 323 K. The equilibrium liquid phase compositions of each system was measured, the corresponding equilibrium solid phases were identified by chemical analysis and X-ray powder diffraction, and the equilibrium phase diagrams were drawn. No double salts or solid solutions were found in the ternary systems BaCl2 + SrCl2 + H2O and KCl + BaCl2 + H2O at 323 K and the phase diagrams both contain one invariant point, two isothermal solubility curves and two crystallization regions. The hydrate forms in the quaternary system KCl + BaCl2 + SrCl2 + H2O at 323 K are the same as those of its subsystems. The quaternary phase diagram contains one invariant point, three isothermal solubility curves and three crystallization regions, where the equilibrium solid phases are KCl, BaCl2·2H2O and SCl2·6H2O. These results provide basic thermodynamic data for the design of a comprehensive utilization process of potassium, barium and strontium resources in gas field water. [ABSTRACT FROM AUTHOR]