Automatic transmission is the core component of all kinds of vehicle transmission systems. Under the trend of vehicle lightweight development, automatic transmissions are required to meet the requirements of high load-carrying capacity and small volume, which leads to increased assembly difficulty. Digital twin (DT) technology, as an effective means of integrating information from the physical world, plays an important role in realizing intelligent manufacturing. The application of DT technology can realize high-quality and high-efficiency assembly of automatic transmission and comprehensively improve the performance of automatic transmission. In this paper, the assembly process of automatic transmissions based on DT is designed, simulated, and tested. DT technology provides data analysis and evaluation capability for workshop production, which enables simulation and full-view monitoring of the assembly process. The research is of great significance to improve the assembly efficiency and assembly quality of automatic transmissions.