The regional economy, population and ecology are closely linked in regional development, and the study of the harmonization between these systems contributes to the sustainable development of cities. In this study, 43 cities in the Bohai Rim region (BRR) urban agglomeration of China were selected for investigation in 2000, 2010, and 2020. By introducing an ecological subsystem indicator (Gross Ecosystem Product (GEP)) applicable to the study area, a coupled coordination degree (CCD) model relating ecology, economy and population was developed to explore the mechanism of sustainable development among the considered cities. The results indicated the following: the BBR GEP increased significantly over the study period, and Beijing and Tianjin exhibited the highest GEP values. The obvious increase in the CCD demonstrated that coordinated development of the BRR has improved since 2000, the center of gravity of coordination has shifted southwards, and stable high-high agglomeration occurs. This study is important for optimizing the strategic layout of regional development, enhancing regional synergy, and promoting high-level and high-quality regional development.