Based on the requirement of the new technology for producing potassium sulfate and N-Mg compound fertilizer, boussingaultite, by the reaction of the mineral shoenite from Kunteyi Salt Lake, Qinghai province, and the industrial by-product ammonium sulfate, the solubilities of the quaternary system (NH4)2SO4-MgSO4-K2SO4-H2O at 25.0oC in the isothermal evaporation and crystallization process were measured using the isothermal evaporation method, and the corresponding phase diagrams were plotted. According to the diagram, this system contains six saturation points and six solid phase fields of crystallization, which correspond to (K1-m,(NH4)m)2SO4, (NH4)2SO4·MgSO4·6H2O, K2SO4·MgSO4·6H2O, MgSO4·6H2O, (K1-n,(NH4)n)2SO4·MgSO4·6H2O and MgSO4·7H2O, respectively. By analyzing and calculating the isothermal evaporation and dissolution phase diagram of this quaternary system at 25.0oC, K2SO4 and (NH4)2SO4·MgSO4·6H2O can be separated via K2SO4·MgSO4·6H2O and (NH4)2SO4 as raw materials. Theoretical calculations about the proposed process were carried out and verified by experiment, which indicated that the yield of potassium sulfate was improved and the magnesium resources were fully utilized.