Many manufacturers today are striving to offer high value-added Product/Service System (PSS) due to increasing competition and environmental pressure. PSS design activities face a variety of challenges such as a high level of customization as well as its resulting challenges, i.e., hidden requirements in product use phase, potential conflicts of design attributes, and internal complexity of service processes. However, existing insights for PSS customization are fragmented and insufficient to support manufacturers. Thus, it is necessary to develop a systematic and comprehensive support to solve those issues. In order to support PSS customization in early design phase, this paper proposes a design framework that involves a design process. The proposed design framework is module-based and thus flexible according to the user needs. In addition, it takes advantage of some existing methods. A case study of elevator PSS design shows the feasibility and potentials of the design framework and its associated design process to its broad usage in industry.