This paper discusses the key contents and technologies of the assembly process design system based on the assembly digital twin model, aiming at the problem that the single digital model exposed in the implementation of digital assembly technology of aviation mechanical and electrical products cannot fully reflect the features, functions and performance indicators of the products. This paper points out the new requirements of the digitization of assembly process design for product digital model, and expounds the definition and development status of digital twin technology,  and discusses the application of digital twin technology in product assembly process modeling. This paper discusses the virtual and real fusion and data exchange of assembling digital twin model and product prototype, and how to use real-time data to aid process decisions and dynamic modification process plan. Finally, the assembly process design can quickly respond to production changes.