At low temperature of 4℃,the inorganic origin of monohydrocalcite (MHC) in Lake Nam Co was investigated by simulating the mainly ionic species and controlling the Mg/Ca ratio in the reaction system. The experimental result indicates that MHC is transformed from amorphous calcium carbonate (ACC), and the formation and stability of MHC are regulated by Mg²⁺ in the reaction solution. When Mg/Ca ratio<2 in the reaction solution, calcite and magnesian calcite are formed, and when Mg/Ca ratio>2 in the solution, MHC is formed, and the existence time in the solution increases with the Mg/Ca ratio. In the case of Mg/Ca ratio being 2, MHC exists in the solution for 48 h, then it transforms to aragonite and a small amount of magnesian calcite. The effect of Mg²⁺ in the formation of MHC lies mainly in inhibiting the nucleation of calcite, so that MHC can be formed and stable in the solution. The results could provide a certain basis for the study of inorganic origin of the MHC in Lake Nam Co.