The metastable phase equilibrium of the quaternary system K+, Mg2+, Ca2+//Cl--H2O at 298.2 K is done using an isothermal method. The equilibrium liquid phase compositions, densities and refractive indexes of the equilibrated solutions are measured at the same time experimentally. The crystalloid forms of the solid phase are determined using an X-ray diffraction method. Experimental results show that this is a complex system at 298.2 K with two double salts (carnallite KCl.MgCl2 center dot 6H(2)O and tachyhydrite 2MgCl(2)center dot CaCl2 center dot 12H(2)O). The metastable phase diagram of this system under the investigated temperature contains three invariant points, five crystallization fields and seven univariant curves. With the changes of J(CaCl2), the water content, densities and refractive indexes change regularly.