This paper introduces the natural enviorenment, geological evolution nd soda-forming mechanism of the Hetong Qahan Nur Soda Lake in Inner Mongolia. It is suggested that in the initial dilute groundwater recharging the lake, the 2(Ca2++Mg2+)/HCO3- molar ratio should be close to 1 (0. 96), which is disadvantagous to the formation of soda brines. But during the evolution from dilute water to high-concentrated soda brine, there are obvious gains of Na+ and HCO3- + CO32-. Drought-enduring and halotolerant (alkaliotolerant) plants which are rich in Na in their bodies may contribute much to the gains.