The thermodynamics phase equilibria of the aqueous ternary systems (Mg(H2PO2)(2) + NaH2PO2 + H2O) and (Mg(H2PO2)(2) + MgCl2 + H2O) at 323.15K were investigated via the classical isothermal solubility equilibrium method. The solubilities and densities of the equilibrated solution were determined. The species of solid phase were confirmed by using the Scherinemakers wet residue method and with the aid of X-ray powder diffraction testing. In accordance with the experimental data, the phase diagrams and density versus composition diagrams were constructed. Both ternary systems are of complex type with three-species solid-salts of [Na(H2PO2)(2)center dot H2O, Mg(H2PO2)(2)center dot 6H(2)O, and double-salt NaMg(H2PO2)(3)], and four-species solid-salts of [hydrate Mg(H2PO2)(2)center dot 6H(2)O, MgCl2 center dot 6H(2)O, dehydrate Mg(H2PO2)(2), and double-salt species Mg-2(H2PO2)2Cl(2)center dot 6H(2)O] formed respectively in the corresponding system. The solubility of Mg(H2PO2)(2) is very low in Na(H2PO2)(2) aqueous solution but large in MgCl2 aqueous solution. Mg(H2PO2)(2)center dot 6H(2)O will dehydrate and transform into Mg(H2PO2)(2) in the concentrated MgCl2 aqueous solution.