Beginning from analyzing city ecosystem, the article discusses connotation, character and balance principle of city ecosystem. On the basis of consulting both domestic and overseas materials data, the rule of city ecosystem is analyzed. So it concludes that city ecosystem is got-up and brittle ecosystem, the substance and energy circle as well as provision of city ecosystem depend on other places. Therefore city ecosystem has a lower capability when it meets an emergency. Human being should research the character of city ecosystem, prevent disjustment of city ecosystem, and set up new balance, create good living environment by using science theory.The paper also discusses the balance and the control mechanism of city ecosystem.