The component solubilities of the HCl–MgCl 2–H 2O system at 40 ∘C were calculated by using Pitzer’s ion-interaction model and the solubility equilibrium constant of HCl⋅MgCl 2⋅7H 2O at 40 ∘C was evaluated according to the solubility data for the HCl–MgCl 2–H 2O system at −19.8, 0, 20, 25 and 50 ∘C. This study can provide the parameters necessary for solubility prediction for the HCl–LiCl–MgCl 2–H 2O system at 40 ∘C and supply a theoretical basis for the manufacturing process which was proposed by Gao and employed to extract MgCl 2⋅6H 2O from salt lake brine.