Abstract: Phase equilibrium and equilibrium solution of in the quaternary system Li<SUB>2</SUB>B<SUB>4</SUB>O<SUB>7</SUB>-Li<SUB>2</SUB>SO<SUB>4</SUB>-LiCl-H<SUB>2</SUB>O at 288 Kwere studied by isothermal equilibrium method and solubilities and densities of the equilibrium solution of this system were determined.Results showed that this quaternary system was simple eutectic type and there was no formation of double salt or solid solution.Based on the solubility data,phase diagram of the system was plotted,which consisted of three univa-riant curves&nbsp; E<SUB>1</SUB>-E,E<SUB>2</SUB>-E,and E<SUB>3</SUB>-E,one invariant point E and three crystallization fields:Li<SUB>2</SUB>B<SUB>4</SUB>O<SUB>7</SUB>·3H<SUB>2</SUB>O,Li<SUB>2</SUB>SO<SUB>4</SUB>·H<SUB>2</SUB>O and,LiCl·H<SUB>2</SUB>O.Experimental results showed LiCl·H<SUB>2</SUB>O had very strong salting-out effect on Li<SUB>2</SUB>B<SUB>4</SUB>O<SUB>7</SUB>·3H<SUB>2</SUB>O and Li<SUB>2</SUB>SO<SUB>4</SUB>·H<SUB>2</SUB>O.Density transformation rule was briefly discussed.