The stable phase equilibrium of mixed aqueous electrolyte system Li+, K+, Rb+, Mg2+//borate-H2O has been studied using the isothermal dissolution method at T = 348.15 K. The space diagram, the planar projection diagram (saturated with Li2B4O7), water content diagram, and diagrams of the physicochemical properties (densities and refractive indices) have been constructed based on the Janecke method. The crystallographic form of the solid phase at the quinary system invariant point was identified by the X-ray diffraction method. Results show that the quinary system belongs to a simple type without solid solution or double salt formed. The crystallographic form of borates formed in this work are Li(2)B(4)O7 center dot 3H(2)O, K2B4O3(OH)(4)center dot 2H(2)O, RbB5O6(O())4 center dot 2H(2)O, and MgB4O5(OH)(4)center dot 7H(2)O. Under the condition of Li2B4O7 saturated, the crystallization area of salts decrease in the order of RbB5O6(OH)(4)center dot 2H(2)O > MgB4O3(OH)(4)center dot 7H(2)O > K2B4O5(OH)(4)center dot 2H(2)O.