By using isothermal dissolution method, phase equilibrium of the quaternary system Li+, Rb+, Mg2+ // Borate-H2O at 298.2 K was investigated. The solubility, density and refractive index of equilibrated solution were measured experimentally. Based on the experimental data, the stable phase diagram, the density vs J(Rb2B4O7) diagram, water content vs J(Rb2B4O7) and refractive index vs J(Rb2B4O7) diagram were constructed. Results indicate that the quaternary system at 298.2 K is of a simple type, its phase diagram consists of one invariant point, three univariant curve and three crystalline phase areas. The size of crystallization areas of salt is in the order MgB4O7 center dot 9H(2)O > RbB(5)8 center dot 4H2O > Li2B4O7 center dot 3H(2)O, which demonstrates MgB(4)O(7)l center dot 9H(2)O can be more easily separated from the solution in this quaternary system at 298.2 K. The comparisons between the stable phase diagrams at 298 K and 348 K show that with the decrease of temperature, the precipitation of rubidium borate becomes easier, while that of lithium borate becomes more difficult. With the increase of J(Rb2B4O7), the water contents decrease obviously, the refractive index and the density increase obviously on the univariant curve cosaturated with RbB5O8 center dot 4H(2)O and MgB4O7 center dot 9H(2)O.