Phase equilibria of the reciprocal quaternary system Li+, Mg2+//SO2-4, B4O2-7- H2O at 288K were studied by isothermal solution equilibrium method. Solubilities and densities of the reciprocal quaternary system were determined experimentally. According to the experimental data, the equilibrium phase diagram and water content diagram of the quaternary system were plotted，respectively. The experimental results show that the quaternary system at 288 K has two invariant points F1 and F2, five univariant curves E1F1, E2F2, E3F1, E4F2, and F1F2, and four fields of crystallization corresponding to Li2B4O7 · 3H2O， Li2SO4 · H2O，MgB4O7 · 9H2O and MgSO4 · 7H2O.