The solubilities and densities of solid-liquid equilibria in the quaternary system KCl-KBr-K2SO4-H2O at 348 K were determined experimentally by using an isothermal solution saturation method. According to the experimental data, the phase diagram of the quaternary system was plotted. In the equilibrium phase diagram of the quaternary KCl-KBr-K2SO4-H2O system at 348 K, there are one univariant dissolution curve and two crystallization fields corresponding to K(Cl, Br) and K2SO4, and no invariant point has been found in this quaternary system. The quaternary system contained solid solution K(Cl, Br). The density values in the equilibrium solution increase with the increase of potassium bromide concentration and decrease with the increase of potassium chloride concentration.