The knowledge of urban metabolism process is a major step towards the design of sustainable development schemes and environmental management. This study systematic simulated and analyzed the mechanisms of Beijing urban ecosystem from a thermodynamic point of view. This assessment model review and compile existing data and studies on environmental issues available primarily at some sources, which include resource accounting and environmental impact assessment. The direct and indirect emergy demand was assessed based on airborne and waterborne pollutants dilution patterns, and concepts of Life Cycle Impact Assessment followed the DALY and PDF methods. Results pointed out (1) the development of economy in Beijing was closely correlated with the consumption of the nonrenewable resources and exerting rising load on environment; (2) of the total emergy use by the economic system, the imported nonrenewable resources from other province contribute most with increasing use from imported nonrenewable resources; (3) the rapid growth of society fixed capital investment drave Beijing's economic development and GDP'S growing; (4)emissions greatly reduced the sustainability of the urban metabolic system by pulling resources for damage repair and for replacement of lost natural and human-made capital. Such a knowledge is a necessary pre-requisite to perform a reliable cost-benefit evaluation of urban sustainability strategies, and provide guidance to policy decisions.