In order to establish the theortetical basis and optimize the production conditions for the separation process of the H_3BO_3-MgSO_4 mixed solution by using Mg(NO_3)_2 as the salting out agent.The phase diagrams of H_3BO_3─MgSO_4─Mg(NO_3)_2─H_2O system at 25℃ and 100℃ were studied.The results show that there appear MgSO_4·7H_2O

MgSO_4·6H_2O

Mg(NO_3)_2·6H_2O and H_3BO_3 three crystallization regions in the phase diagram at 25℃

and three crystallization regions of MgSO_4·H_2O

Mg(NO_3)_2·2H_2O

and H_3BO_3 at l00℃.When the temperatures rise from 25℃ to 100℃

the crystallization regions for magnesium sulfate hydlates grow larger and while that of H_3BO_3 get smaller.Only MgSO_4·4H_2O could be crystallized out alone

if evaporating the H_3BO_3─MgSO_4 solution in the presence of Mg(NO_3）_2.On cooling the MgSO_4 mother liquor

H_3BO_3 can be separated alone without any addition of water.