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Maintaining urban systems consumes a large amount and variety of materials and leads to waste flows. Carbon is a basic element that intuitively characterizes the metabolic characteristics of urban resource consumption and pollution emission. In this study, we integrated the carbon metabolic flows among 18 metabolic actors and between these actors and the atmosphere, and calculated flows of material in many categories using empirically derived coefficients to estimate the associated carbon flows (emission and absorption). Taking Beijing as an example, we analyzed the dynamic changes in the carbon metabolism and the structural characteristics of material utilization. We defined two indices to characterize the metabolism (the carbon imbalance and external dependence indices), and identified key actors responsible for changes in the indices. The total carbon metabolism (inputs and outputs) increased by 64% and 200%, respectively, from 1995 to 2015, mainly driven by energy consumption, which accounted for more than 78% of the total. In addition, input growth was driven by food, accounting for up to 6% of the total. The carbon imbalance and external dependence indices increased to nearly two and four times their 1995 values, respectively, mainly due to the Manufacturing, Electricity and Heat Production, and Energy Conversion actors' demand for food or energy during the early part of the study period, and by the rapid growth of food or energy required by the Urban Life and Transportation actors. Identifying and comparing the key metabolic actors provided a novel way to analyze data to determine targets for carbon regulation and emission reduction measures for Beijing.