The sustainability evaluation on the clean development mechanism (CDM) project of biomass direct combustion power generation required quantitative analysis from the aspects such as economic, social and environmental benefits. The emergy analysis of the CDM project was conducted and its emergy sustainability evaluation indexes were established with the emergy theory. As the case of 25 MW biomass direct combustion power generation CDM project in Central China, the system’s sustainable performance was evaluated. The results showed that the CDM project could bring the benefit of 2.51 million US dollars annually for greenhouse gas (GHG) emission reduction. Because the benefit effectively increased the emergy yield ratio and reduced the environmental load of the power generation system, the system had not only been higher competitiveness, the emergy sustainability indexes of the system could be increased to 6.45 which can cause the system to be in the potential and sustainable development process for a long time.