Based on the thermodynamic formulation of Darwin's Theory of Survival of the Fittest, this paper introduces the thermodynamic concept Exergy into an ccadynamic model for lakes as a goal function of some important model parameters, then the ecological properties of lake ecosystem are considered extensively. The changes in model parameters with time are aecomplished by using the modelling procedure of parameter combinations under exergy control, then the changes in the species composition and ecological structure of ecosystem are reflected by the varying parameters. This improvement overcomes the original ecodynamie model's defects of strong rigidity, poor adaptability and inadequate considerations of ecosystem properties. The modelled results of Di-anchi Lake ecosystem have shown that the improved ccodynamic model is satisfactory and practicable.