Abstract: There are rich salt lake resources in Qinghai-Tibet plateau of China，which were known as the abundant borate resources and large reserves of potassium and magnesium to the world.Therefore，it is very important to carry out deep research on metastable phase equilibrium of boron-containing polycomponent in the exploitation of boron and magnesium resources from salt lakes in China.An experimental study on metastable equilibria at 288 K in the MgB↓4O↓7-K↓2B↓4O↓7-Na↓2B↓4O↓7-H↓2O quaternary system has been done by means of isothermal evaporation method.Furthermore，the solubility and density of the metastable equilibrium solution were determined.The results showed that the system was of simple eutectic type and there′s no complex salt formed in the MgB↓4O↓7-K↓2B↓4O↓7-Na↓2B↓4O↓7-H↓2O quaternary system at 288 K.The metastable equilibrium phase diagram was plotted by referring to the experimental data.In the phase diagram of the quaternary system，there were one invariant point(E)，three univariant curves(E↓1-E，E↓2-E，E↓3-E) and three fields of crystallization (MgB↓4O↓7·9H↓2O，Na↓2B↓4O↓7·10H↓2O and K↓2B↓4O↓7·4H↓2O).From the diagram，K↓2B↓4O↓7 had the salting-in effect on Na↓2B↓4O↓7.