Abstract: A laboratory digital twin production line for assembling smart phones is built as a prototype testing platform to study the yield transmission and formation mechanisms of production lines caused by the causal relationship of factors in multiple assembly stages. Focusing on two yield issues in screw locking and dispensing processes, a coupled modeling method integrating product quality and equipment reliability for smartphone assembly processes is proposed with the consideration of causal relationships. A dynamic Bayesian network (DBN) model is developed considering causal factors across multiple assembly stages. Then, root cause tracing and importance evaluation affecting final yield formation are conducted. The feasibility and effectiveness of the proposed approache are tested and verified on the digital twin testing platform for assembling smart phones, providing performance evaluation support for yield loss prevention mechanisms and proactive maintenance decisions on production lines.