Analysis of the mapping and transferring mechanism, and the sensitivity of quality characteristics of Complicated Electromechanical Products (CEP) reveal that the coupling of functionality and structure, and uncertainty in their mapping relations are important factors that influence the quality of CEP. By integrating the principals of Axiomatic Design (AD), this paper proposes a concept of decoupling design for quality characteristics of CEP. A model for decoupling control in the mapping processes of CEP is presented. In the model, formation-process of quality characteristics of CEP is transformed into iterative zigzagging decomposition and mapping between functional-domains. Then mapping matrixes are utilized to facilitate product designers to identify coupled quality characteristics in CEP, and to reduce complexity in function and physical structure and the product development. The model presented is used in the design of the transmission system in the first direct-drive hobbing machine in China, which effectively reduces the complexity of transmission system of hobbing machine and improved gear hobbing accuracy.