The solubility data of the quaternary system Na+, NH4 +, Mg2+∥SO4 2––H2O and its subsystems Na+, NH4 +∥SO4 2––H2O and Mg2+, NH4 +∥SO4 2––H2O were measured by an isothermal dissolution method at T = 318.15 K. The physical property data (density and viscosity) of the supernatant were also obtained. The experimental results show that both ternary systems contain two invariant points, which are complex systems. The quaternary system (Na+, NH4 +, Mg2+∥SO4 2––H2O) has four invariant points, nine univariate curves, and six crystallization regions. The crystal area of double salt (NH4)2Mg­(SO4)2·6H2O is the largest, and it is the easiest to crystallize from the saturated solution. Density and viscosity varies regularly with solution composition in each system. The translation method is used to predict the quaternary system, and the results are consistent with the experiment. Based on the phase diagram of Na+, Mg2+, NH4 +∥SO4 2––H2O, the process of producing a nitrogen–magnesium compound fertilizer was designed, which provided a theoretical basis for guiding the industrial crystallization of salt lakes.