The stable phase equilibrium data of the K+, Mg2+//Cl-, SO42- + H2O quaternary interaction system and its four ternary subsystems KCl-K2SO4-H2O, KCl-MgCl2-H2O, K2SO4-MgSO4-H2O, and MgCl2-MgSO4-H2O were determined by an isothermal dissolution equilibrium method at 353.15 K, and the corresponding stable phase diagram of the above systems was plotted. By analyzing the phase diagrams, eight invariant points and nine crystallization regions were confirmed in the quaternary system K+, Mg2+//Cl-, SO42- + H2O. Based on the extended Pitzer model, the solubility data of the ternary subsystems and the quaternary system were calculated. The results showed that the calculated values were close to the experimental values. According to the comparison between the stable phase diagram of the quaternary system at 308.15 K and 353.15 K, a new process of preparing potassium sulfate at 353.15 K and crystallizing picromerite (K2SO4 center dot MgSO4 center dot 6H(2)O) at 308.15 K by using potassium chloride and magnesium sulfate as raw materials was determined in this paper, and the process route was verified. As a result, the maximization of raw material utilization rate was achieved, and a new way for the comprehensive utilization of salt lake resources was put forward.