Construction manipulator is the key component of construction machinery to complete the operation task, thus its assembly link directly affects the product quality and operation performance of the whole machinery. In order to solve the problems of low assembly efficiency and the inability to fully reflect the assembly process indexes and product characteristics in the traditional construction machinery arm assembly, this paper studies assembly process modeling and simulation for construction machinery arm based on assembly data and digital twin. By extracting and processing the assembly resource data and field measurement data of mechinery arm, the assembly process information database under the digital twin environment is constructed, which lays the foundation for the virtual assembly model construction of mechinery arm. Through the real-time data interaction between virtual space and physical space, a complete assembly digital twin space is formed. Finally, taking the assembly line of an excavator arm as an example, it is shown that the digital twin-based assembly simulation can monitor the assembly process in real time and optimize its configuration to improve assembly efficiency. Thereby, an effective closed-loop feedback mechanism is constructed for the whole assembly process of construction machinery arm.