Abstract: This paper shows an elementary attempt to model the water-magnesian calcite system by making use of the up-to-date achievements in the researches on magnesian calcite and water-calcite system. Then, on the basis of this system, a general lake model was developed which includes climatic, hydrologic and hydrochemical conditions. The lake model is applied to the quantitative study on the phenomenon of the alternating carbonated and clastic rich rhythmic deposition in lakes controlled by seasonal climatic change. The evolution process of the lacustrine hydrochemistry producing rhythmic deposition of the above style is demonstrated in this paper. It is verified that the carbonate deposits in the rhythm might be formed in summer of each year.