An isothermal dissolution method was employed to investigate the phase equilibrium at 308 K of the mixed aqueous system containing lithium, magnesium, sulfate, and borate. The solid phases of the invariant points were confirmed with the X-ray diffraction method. The stable phase diagram, the water content diagram, and the diagrams of the physicochemical properties versus the composition of the solution have been constructed. The system contained two invariant points (noted as E-1 and E-2), five univariant curves, and four crystallization fields corresponding to single salts epsomite (MgSO4 center dot 7H(2)O), hungchaoite (MgB4O7 center dot 9H(2)O), lithium sulfate monohydrate (Li2SO4 center dot H2O), and lithium borate trihydrate (Li2B4O7 center dot 3H(2)O). Invariant points E-1 and E-2 are of commensurate invariant points. The crystallization fields decrease in the sequence MgB4O7 center dot 9H(2)O, Li2B4O7 center dot 3H(2)O, Li2SO4 center dot H2O, and MgSO4 center dot 7H(2)O, which was contrary with the solubility of the salts. Compared with the temperature at 298 K, the crystallization area of salts MgB4O7 center dot 9H(2)O and Li2SO4 center dot H2O enlarged, whereas that of salts Li2B4O7 center dot 3H(2)O and MgSO4 center dot 7H(2)O decreased at 308 K, which lead the change for property of invariant point, one invariant point change from incommensurate to commensurate. The solubility of salt MgSO4 increases with temperature strongly; thus, its crystallization field decreases obviously, whereas the solubility of salt Li2SO4 decreases with temperature and its crystallization field is enlarged.