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Underground brines are important liquid mineral resources, and the phase equilibria of the salt–water system on underground brines are conducive to the development and utilization of the rich mineral resources in brines. The phase equilibrium studies of the quaternary systems NaCl–MgCl2–SrCl2–H2O and KCl–MgCl2–SrCl2–H2O at 298.2 K were carried out using the isothermal dissolution equilibrium method. The solubility of each salt in the equilibrium liquid phase was measured, and the corresponding equilibrium solid phases were identified by XRD. The results showed that the quaternary system NaCl–MgCl2–SrCl2–H2O contained five isothermal solubility univariate curves, two invariant points, and four equilibrium solid-phase crystallization regions (SrCl2·2H2O, SrCl2·6H2O, MgCl2·6H2O, and NaCl). The phase diagram in the quaternary system KCl–MgCl2–SrCl2–H2O has three invariant points, seven isothermal solubility univariate curves, and five equilibrium solid-phase crystallization regions (SrCl2·6H2O, SrCl2·2H2O, MgCl2·6H2O, KCl·MgCl2·6H2O, and KCl). The phase equilibrium relationships of these two quaternary systems at different temperatures were also analyzed detailedly. Furthermore, the Pitzer model was used to calculate the solubilities of salts in the above systems. The calculations and experimental results are compared, and the calculated phase diagram agrees well with the experimental phase diagram.