Abstract: The moderm ecosystem which centres on mankind and biology is a comprehensive combination of natural ecosystem and social system. It is a system of live motion which results from the exchange of matter and energy between the system and its surroundings. The second law of the thermodynamics studies essentially the exchange of matter and energy between the system and its surroundings; this exchange is the ultimate motion in the ecosystem. Thus in the view of the thermodynamics the exchange of matter and energy between ecosystem and its surroundings will inevitably cause entropy changes of the ecosystem. The author tries in this paper to discuss the relation between entropy and ecosystem from the viewpoint of the theory of non-equilibrium thermodynamics and decide the direction of the irriversible process by entropy changes, expounding the objective law of the existance, development and death of living beings in the ecosystem