## 1 Introduction  
Integrating design-manufacturing-operation and maintenance (O&M) of complex products is critical for intelligent manufacturing. Complex products refer to products that use high technology, complex processes, a large number of parts, a large number of levels of product structure, a large number of changes in the relationship between parts and products, and at the same time, have a high value, such as large ships, high-speed trains and carrier rockets, etc. They have the characteristics of interdisciplinary, multidepartmental collaboration and high integration [1]. The manufacturing mode of complex products is small-batch, multivariety and customized. From the perspective of engineering technology or management, due to its large number of subsystem components and complex system composition relationships, the whole process--from design to manufacturing as well as operation and maintenance--has extremely high complexity and difficulty in its operation [2; 3; 4]. Low information transparency in the design, manufacturing and O&M of complex products results in department barriers. With the application of a new generation of internet technology, communications technology, IoT technology, etc., in the traditional manufacturing industry, the manufacturing industry of complex products continuously advances toward the intelligent direction [5; 6; 7]. Additionally, the manufacturing industry of complex products confronts more and more urgent, large-scale, customized customer requirements; therefore, the intelligent manufacturing model continues to face new challenges [8]. In this context, various countries launched the corresponding intelligent manufacturing development strategy; for instance, Germany proposed the \"Industry 4.0\" [9], the US proposed the \"industrial internet\" [10], China proposed the \"Made In China 2025\" [11], etc. These strategies focus on the deep integration of physical information, including the deep integration of physical information for the design, manufacturing and O&M of a complex product as an important part [12]. Currently, each link of design, manufacturing and O&M of product is mutually independent. A significant amount of data generated in each link cannot be interconnected, resulting in long product design cycles, abnormal manufacturing links, and low O&M service response. Therefore, it is urgent to achieve a collaborative manufacturing mode of design, manufacturing and O&M business integration. Digital twin, a key technology of intelligent manufacturing, provides the possibility for complex product design, manufacturing and O&M integration [13]. The core idea of digital twin originates from the NASA and focuses on predicting aircraft service life through data [14]. Prof. [PERSON] proposed the concept of digital twin in 2003, i.e., the virtual equivalent of the physical system. It is called an \"ideal conceptual model of life cycle management of product\" [15]. The digital twin can be defined as the digital representation of physical entity in virtual space and accurately matches the operation of a physical process happening in real-time [16]. Therefore, digital twin is a very practical tool, and through which the current state and future state of a product can be understood via the synchronization of physical mirroring and entity. Currently, many scholars are researching digital twin technology in many aspects. [PERSON] et al. proposed a universal digital twin system and body concept system to promote the standardized development of digital twin [17]. [PERSON] et al. proposed the procedure for the quantitative calculation of digital twin performance, and the KPI used to confirm the digital twin flexibility, which provides a guarantee for the researchers in the development of the digital twin process [18]. [PERSON] et al. proposed a 5D architecture model of digital twin, which is mainly composed of a physical entity, virtual entity, service module, twin data module, and linking module.