The metastable phase equilibria of the quaternary system Li+,K+parallel to Cl-, and SO42--H2O was studied at 273.15 K using an isothermal evaporation method. The solubilities and densities of the euqilibrated solution were determined. According to the experimental data, the metastable phase diagram, the wate-content diagram, density diagram, and pH value diagram were plotted. This quaternary system is of a complex type; the double salt KLiSO4 was found at the studied temperature. The metastable phase diagram consists of three invariant points seven univariant curves, and five salt crystallization regions. Potassium sulfate has the largest crystallization field and can be easily separated from the solution in this system at 273.15 K using an evaporation method. Comparisons between the metastable and the stable phase diagram at different temperatures show that the crystallization forms of the salts are the same , whereas the crystallization field of salt K2SO4 becomes obviously larger with the temperature decrease. If more pure potassium sulfate needs to be separated as much as possible, a fall in temperature is necessary.