As a concentrated area of material throughput, cities are often faced with some problems of resource and environment. The Beijing-Tianjin-Hebei region has the largest economic scale in northern China, so it is bound to be burdened with huge pressure on resources and environment. From the perspective of metabolism and based on the calculation of urban material input and output, this study selected the Beijing-Tianjin-Hebei region as an example, used the ecological network analysis method to deeply explore the specific path and node relationships within the metabolic system, and analyzed the changes from time. The results showed that 97% of the flow amount was within the cities, indicating the need for cities to strengthen their complementarity. The total metabolic flow of 13 cities in the Beijing-Tianjin-Hebei region increased by 3.3 times that of 2000 in 2017, with Chengde, Qinhuangdao and Tangshan growing the most rapidly, while Hengshui was the only city with a decrease. Among the key paths of each city, 71.8% showed an upward trend of flow, and a high degree of sectoral concentration. Manufacturing became one sector most involved in the key paths, and manufacturing-environment was the key path type that existed in most cities. The exploitation and control were the dominant relationship type in the 13 cities. From 2000 to 2017, the positive change of ecological relationship was less than the negative change, which may require breakthroughs in key sectors such as transportation, mining and manufacturing. The identification of metabolic characteristics and their changes, such as key path and ecological relationship, effectively opened the "black box" of urban metabolism, and reliable theoretical basis for alleviating the resource and environment problems caused by urban material utilization.