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The difficult separation of magnesium–lithium has always been a problem that impedes the comprehensive utilization of salt lake brine resources. In this paper, a method for the separation of magnesium and lithium based on the crystallization of magnesium sulfate at high-temperature supersaturation and a low viscosity was investigated. The microstructure of soluble solutions was analyzed, and the results showed that, in a single-salt solution, controlling the temperature can change the contact ion pair structure of MgSO₄ solution, and the arrangement of SO₄²⁻ and H₂O in the second hydration layer changes. In the Li₂SO₄ solution, the hydrogen bonds between SO₄²⁻ and H₂O break, and the surrounding water structure changes, breaking the similarity of the microstructure of magnesium–lithium and enhancing the separation effect. In a multi-ion system, the change in water structure in the solution decreases with the increase in Cl⁻ concentration. Controlling the temperature of salt lake brine with different magnesium–lithium mass ratios, it was found that the magnesium–lithium mass ratio in the brine could be reduced by one-third; when the magnesium–lithium mass ratio was 10:1~160:1, the loss of lithium could be controlled within 5%, but when the magnesium–lithium mass ratio was 5:1, the loss of lithium was 25.06%. The main reason for lithium loss is that Li₂SO₄ in the liquid phase enters the solid phase as a cluster and is entrapped during the MgSO₄ crystallization process. The entire experiment shows that controlling the temperature process is more suitable for salt lake brine with a high magnesium–lithium ratio.