The quaternary water-salt system RV, Mg2+// Cl-, borate -H2O is an important subsystem of the composition of Pingluo underground brine, Sichuan province. An isothermal dissolution equilibrium method was used to study the phase equilibrium of the system, and the solubilities, densities, and refractive indices of the equilibrated solution of the system were determined. On the basis of the solubility data, the isothermal dissolution phase diagram, the water figure, the diagrams for the density-composition and refractive index -composition were obtained. Results suggest that the quaternary system is of a complex reciprocal system with the formation of double salt RbCl center dot MgCl2 center dot 6H(2)O. Three invariant points and seven univariant curves divide the phase diagram into five crystalline zones. The five crystalline regions correspond to four single salts, rubidium chloride (RbCl), bischofite (MgCl(2)6H(2)O), rubidium pentaborate tetrahydrate (RbB5O6(OH)(4)center dot 2H(2)O), and hungchaoite (MgB4O5(OH)(4)center dot 7H(2)O), and one double salt rubidium-carnallite (RbCl-MgCl2 center dot 6H(2)O). The size of the RbB5O6(OH)(4)center dot 2H(2)O crystallization field is the largest, whereas that of MgCl2 center dot 6H(2)O is the smallest. Comparisons between the stable phase diagrams at 323 K and 348 K show that there is no change with the crystallization forms of the salts, while the crystallization field of salt RbB5O6(OH)(4)center dot 2H(2)O becomes obviously larger with the increase of temperature; a rise in temperature is beneficial to separate RbB5O6(OH)(4)center dot 2H(2)O from the solution.