Assessing the coordination between the economic development and the environment is a key to the sustainable development of developing countries. Most of the new progresses in modeling the coordination are applied in evaluating the status quo, rather than in predictions. Furthermore, most existing studies are focusing on balancing the current economic development and the environment, neglecting the important factor-the development potential. Therefore, this paper presents the use of Composite Development Index (CDI), which is calculated from not only the current development status, but also the development potential, in the modeling of the economic-environmental coordination. Using the development of Lianyungang as an example, this research also predicted the future coordination between economic development and environment based on the observed development status and the coordination, using a GM(1,1) gray model. The results show (1) the economic-environmental system in Lianyungang changed from the original symbiotic mode to high consumption in environmental resources during 1996 and 1999; (2) 2000-2006 was characterized by a rapid adjustment between the economic development and the environment, with continued increases in CDI and a stable level of coordination; (3) the limits of the environment on economic development will recur between 2007 and 2009; and (4) from 2010 to 2015, both the level of coordination between economic development and environment and CDI will have a rapid increase. Overall, after 1999, the degree of coordination between the economic system and the environment in Lianyungang is below zero, indicating a fast economic development at the expense of the environment, and hence highlighting the needs to balance the future economic development and environmental conservation in the area.