The ternary system of NaH2PO4 + Na2SO4 + H2O at 323.15 K and 343.15 K is studied by the method of isothermal solution saturation. The equilibrium solid phases are investigated by the Schreinemaker's method of wet residues and verified by X-ray diffraction. The solubility of the system is determined. On the basis of the experimental data, the phase diagrams are plotted and the crystallization regions are determined. The phase equilibrium at different temperatures are compared and discussed. This comparison further illustrates that the temperature can influence the equilibrium of the ternary system. The research can offer meaningful data support for separation processes and further theoretical studies.