The stable equilibrium solubility, density, and refractive index of the quaternary system Li+, Na+, Mg2+//SO42--H2O were studied at T = 323 K by the isothermal dissolution method. The results show that at T = 323 K, a sodium-magnesium sulfate mineral astrakhanite (Na2SO4 center dot MgSO4 center dot 4H(2)O) and a lithium-sodium sulfate double salt (Li2SO4 center dot Na2SO4) are formed in this quaternary system 323 K, but no double salt or solid solution formed between lithium sulfate and magnesium sulfate. The phase diagram consists of seven isothermal dissolution curves, five regions of crystallization, and three invariant points, among which one belongs to an commensurate point (H-3) and two belong to incommensurate points (H1 and H-2). Its five crystallization fields correspond to single salts Li2SO4 center dot H2O, Na2SO4, hexahydrite (MgSO4 center dot 6H(2)O), and two double salts astrakhanite (Na2SO4 center dot MgSO4 center dot 4H(2)O) and Li2SO4 center dot Na2SO4. Hexahydrite (MgSO4 center dot 6H(2)O) has the smallest crystal phase region, while lithium sulfate monohydrate (Li2SO4 center dot H2O) has the largest crystal region. At equilibrium, the refractive index and density in the solution change regularly with the change in concentration of sodium sulfate. Comparisons of the phase diagrams of Li+, Na+, Mg2+//SO42--H2O at different temperatures show that the temperature is the main factor for the salt crystals, not only the crystallization form but also the crystal water content.