By means of wet solid phase method，the phase equilibrium relations of Na₂SO₄-MgSO₄-H₂O ternary system at 45 ℃ was studied．The dissolution data of ternary system was measured by isothermal solution equilibrium method，and the phase diagram of the ternary system was drawn．The results show that，the ternary system at 45 ℃ phase diagram has five regions．The regions were sodium sulfate pure crystallization region，sodium sulfate and complex salt of bloedite crystallization region，complex salt of bloedite crystallization region，complex salt of bloedite and heptahydrate magnesium sulfate crystallization region，heptahydrate magnesium sulfate crystallization region.