Confronting intense resource and environmental pressures, China initiated circular economy (CE) strategies and has implemented promoting local practice to approach the challenge. Cities are key policy executors in promoting CE policies. To evaluate a city's CE performance, most studies have adopted only indicators. However, the correlation between CE indicators and the internal socioeconomic metabolic system is unclear but important to understand the formation mechanism of CE performance. Therefore, this study aims to establish a correlation via ecological network analysis and provides an analytical framework to explore the mechanism behind CE performance. Taking 16 cities of Shandong Province as example, a city's ecological network model with 9 sectors was constructed to identify the key factors based on standardized material flow accounting. The results showed that a strong positive correlation presented between urban resource productivity and the economic development level. Recycling industry in the cities with higher recycling rate had a more prominent function in urban system because of their large volumes and the great effects on other sectors. CE performances were mainly affected by the key sectors including manufacturing, mining and internal environment (IE) that have large effects or specific ecological relationships with the recycling industry (RI) and IE. The sectors in mutualism or competition relationships with RI benefitted or inhibited city's CE performance, respectively. The sectors that presented a resource-exploitation relationship or control relationship with IE limited city's CE performance. Our research provides a method for analyzing a city's CE performance from the perspective of socioeconomic metabolism.