The Qinghai Lake is the only modern inland saltwater lake in China where dolomite, calcite and aragonite are reported to coexist in lake bottom sediments. To discuss the influence of magnesium on the polymorphs of calcium carbonate, the authors chose Qinghai Lake water and sterilized Qinghai Lake water as the reaction solution and CaCl₂ and MgCl₂·6 H₂O as the reactants, and precipitated calcium sulfate and calcium carbonate by the method of variable-controlling with different concentrations of Mg²⁺ at laboratory room temperature. When only CaCl₂ was added into Qinghai Lake water, gypsum (CaSO₄·2 H₂O) and vaterite (CaCO₃) were precipitated. However, when CaCl₂ and MgCl₂·6 H₂O were added together into the lake water, gypsum disappeared, and the synthetic products changed to calcium carbonate, including calcite and vaterite. When Mg²⁺ concentration in lake water water increased to 0.62 mol/L, vaterite disappeared, and the products were composed of calcite and aragonite. With the increasing of Mg²⁺ concentration, the content of aragonite rose steadily, while content of calcite decreased gradually. When Mg²⁺ concentration rose to 1.22 mol/L or higher, calcite disappeared completely, only aragonite was precipitated from the solution. These results indicate that the SO₄^2- in lake water inhibited the formation of CaCO₃ under the condition of no or very low concentration of Mg²⁺. However, when extra Mg²⁺ added into the lake water, they could release the inhibition of SO₄^2-, and then Ca²⁺ were free to combine with HCO₃^- and CO₃^2- to form calcium carbonate. In addition, the polymorphs of synthetic calcium carbonate was obviously controlled by Mg²⁺ concentration. With the increase of Mg²⁺ concentration in the lake water, calcite and vaterite were no longer stable, while aragonite was gradually dominant. When Mg/Ca reached 6.1, only aragonite was stable in the reaction product.