The solubilities and refractive indices of the quaternary system (Li2B4O7 + Na2B4O7 + K2B4O7 + H2O) were attained at 323.15 K by the isothermal dissolution equilibrium method. The dry-salt phase diagram, water content diagram and refractive index diagram were plotted. The phase diagram contains three univariant curves, three crystallization fields (Li2B4O7 center dot 3H(2)O, Na2B4O7 center dot 10H(2)O, K2B4O7 center dot 4H(2)O), and one invariant point. The system is a simple quaternary solid-liquid equilibrium type without double salt or solid solution. A comparison of the phase diagram of the quaternary system at 288.15 and 323.15 K shows that the crystallization regions of Li2B4O7 center dot 3H(2)O becomes larger with the increasing of temperature. The refractive indices calculated with semiempirical formula are in good agreement with the experimental results. The brine separation process was demonstrated with the phase diagrams at 288.15 and 323.15 K. All Li2B4O7 center dot 3H(2)O, most of Na2B4O7 center dot 10H(2)O, and K2B4O7 center dot 4H(2)O can be separated from the solution with the phase diagrams at 288.15 and 323.15 K.