Abstract: Water joint assembly is a critical process in the assembly of liquid cooling systems for active phased array radar, with high requirements for assembly quality and sealing. To enhance the intelligence level of water joint assembly, digital twin technology is integrated with water joint assembly robots. A digital twin system for water joint assembly robots has been developed through research and design. First, the steps for automated water joint assembly are studied, followed by an analysis of the architecture of the water joint assembly digital twin system, which includes the physical layer, data-driven layer, twin model layer, and application layer. The system is constructed through the acquisition and processing of multi-source heterogeneous data from water joint assembly robots, the development of digital twin models, and the creation of functional modules. An application verification is conducted using water joint assembly as an example. The constructed digital twin system achieves a mapping between the virtual and physical processes of the entire water joint assembly, enabling real-time monitoring and traceability of the assembly status and screw tightening torque. This increases transparency and improves the intelligence level of the water joint assembly process.