The organic pollution is one of the important questions of the water quality deterioration in Taihu Lake. The natural degradation of COD，approximately representing the total organic pollutants, in the south of Meiliang Bay of the lake was mechanically studied and quantitatively simulated by means of the mechanism of chemical reactions. Under the conditions of constant temperature (20 C and 30 C) and uniform shaking, the one-order reaction process of COD degradation was quantitatively analyzed and simulated on the basis of Arrcnius equations. The rate constants of degradation (k₂-0. 01856d^-1， k₃₀ = 0. 0364d^-1) were calculatcd in 20 C and 30 C. The constants may increase by two times when the temperature increased 10C. As a result，the apparent active energy (E_a) and the factor before index (A) were estimated to be about 4.9 X 10⁷J and 1. 49X10⁷J，respectively. The formula of COD declining of the Meiliang Bay, Taihu Lake was made out：C_t= C₀·exp[-1. 49 X 10⁷e^-4.9Xl04/RTt].