Abstract: This Paper,based on the energy analysis of Xishan Island of Tai Lake in Jiang-su Province, presents a general procedure and the methods for studying a regional ecosystem by energy analysis approach. We have analysed the energy flow of the natural subsystem, agricultural subsystem and industrial subsystem in this island respectively and integrated these in an overall diagram of energy flow. Results of this study show that the deterioration of the vegetation and the environmental quality in the area is induced by improper consumption of the large amount of imported energy. We conclude that the energy analysis approach can serve as a guidline for the regional constriction planning.