Methodologies for the identification of key characteristics have been widely applied in quality management through the selection of critical dimensions and the measurement of variations. However, methods for both the identification and decomposition of key characteristics have not yet been developed, and more research is still required. In answering this need, a systematic top–down decomposition approach for the identification of key characteristics is proposed. The methodology of the identification and decomposition of key characteristics can be divided into two steps: first is the construction of candidate characteristics, and second is the identification of key characteristics. These steps are based on precise mathematical definitions. Initially, the necessary information for the construction of the candidate characteristics is obtained from analysis based on assembly-oriented graph, and that information is then conveyed utilizing feature adjacency matrix. A concept for a propagation chain is then proposed, and a search algorithm for an auto-generating propagation chain is obtained through feature adjacency matrix. The degree of influence of the candidate characteristics on the key characteristics is then defined. A formula is derived that can calculate the degree of influence by utilizing a variation model. Finally, the process of the identification of the key characteristics is achieved according to the relative degrees of influence. An aircraft boarding gate is presented in order to validate the proposed methodology. Two key characteristics of the aircraft boarding gate are identified, and the results indicate the methodology’s feasibility.