The stable phase equilibrium and phase diagram of the quaternary system K+, Rb+, Mg2+ // borate center dot H2O at 348 K were determined by isothermal dissolution method. The densities, refractive indices, and solubility values of the equilibrated solution in the quaternary system were measured. The crystalloid forms of the solid phase of invariant point were identified by X-ray diffraction method. The stable phase diagram, water content diagram, and the diagrams of the densities/refractive indices versus composition were plotted based on the measured data. Results show that this quaternary system at 348 K without double salt or solid solution formed belongs to a simple eutectic type. The invariant point E saturated with three salts corresponding to potassium tetraborate tetrahydrate (K2B4O3(OH)(4)center dot 2H(2)O), rubidium pentaborate tetrahydrate (RbB5O6(OH)(4)center dot 2H(2)O), and hungchaoite (MgB4O5(OH)(4)center dot 7H(2)O). The crystallization field of RbB5O6(OH)(4)center dot 2H(2)O is the maximum, meaning that it has the smallest solubility among the coexisting salts. On the solubility curves AE (cosaturated with salts MgB4O3(OH)(4)center dot 7H(2)O and RbB5O6(OH)(4)center dot 2H(2)O) and BE (cosaturated with salts K2B4O5(OH)(4)center dot 2H(2)O and RbB5O6(OH)(4)center dot 2H(2)O), the water content decreases, the densities and refractive indices increase with the increase of J(K2B4O7); on the curve CE (cosaturated with salts MgB4O5(OH)(4)center dot 7H(2)O and K2B4O5(OH)(4)center dot 2H(2)O), the water content and the densities increase, the refractive indices decrease with the increase of J(K2B4O7).