Abstract: To investigate the propagation characteristics of ultrasonic waves in complex structures such as inertia friction welded components of aero-engines and to enhance the effectiveness of internal defect detection, a time-domain finite element simulation method based on multiphysics coupling is proposed. A simulation model is developed by combining the theory of ultrasonic wave propagation in fluid and solid domains with the effects of multiphysics wave interaction and water-immersed focused acoustic fields. The model is validated through comparative analysis between simulation results and experimental measurements on standard ring specimens. The comparison demonstrates that the method accurately simulates ultrasonic wave velocity and wave field distribution in complex structures. It is further applied to simulate the ultrasonic inspection process of a specific compressor disk-drum welded component, providing theoretical support for optimizing ultrasonic testing techniques and contributing to improved accuracy and efficiency in the inspection of aero-engine inertia friction welded components.