Using an isothermal evaporation method, the metastable phase equilibrium of the quaternary system Li+, Na+//Cl-, B4O72--H2O was investigated at 273 K. The useful data for solubilities, densities, and pH values were measured, based on which the metastable phase diagram, water content diagram, density vs composition diagram, and pH value vs composition diagram of this system were constructed. The metastable phase diagram of this quaternary system contains two invariant points, five univariant curves, and four crystallization fields. This quaternary system is of a simple cosaturation type, without double salt or solid solution formed. The four crystallization fields are corresponding to salts NaCl, LiCl center dot H2O, LiBO2 center dot 8H(2)O, and Na2B4O7 center dot 10H(2)O, respectively. The sequence of the crystallization field sizes of slats is salt Na2B4O7 center dot 10H(2)O > salt LiBO2 center dot 8H(2)O > salt NaCl > salt LiCl center dot H2O, which demonstrates that the borates, including salt Na2B4O7 center dot 10H(2)O and salt LiBO2 center dot 8H(2)O, can be more easily separated from the solution in this system at 273 K. The crystalloid form of lithium borate is LiBO2 center dot 8H(2)O in the given system at 273 K, instead of Li2B4O2 center dot 3H(2)O formed in the system containing with lithium borate at temperatures above 273 K.