Assessment of city sustainability with the consideration of synergy among economy–society–environment criteria

City sustainability means a balance among economy, society, and environment in the rapid urbanization process. Effective assessment of city sustainability could guide a city to develop in the desired direction. Northeastern China was one of the forefronts of China’s reform and opening up, but now it has become a “sore point” in national development. Liaoning province plays a crucial role in the process of realizing the revitalization of Northeastern China. This paper investigated the sustainability performances of cities in Liaoning by following the process of multi-criteria decision making. A novel weighting method was proposed based on Synergetics that considers the interrelationship among sustainability criteria. The results showed that the sustainability level of Liaoning was not ideal, with only two cities having sustainability performances over 0.55. The cities located in the Middle region had a higher sustainability level than those in the West and East regions. Moreover, the performances on the three dimensions were uncoordinated. The salary of employed staff and disposal income were the two important factors on sustainable development. The forecasting analysis showed that the sustainability level of the cities will develop at a rapid pace in the future. Finally, we put forward targeted suggestions for sustainable development in Liaoning province.