Tungsten alloy, known for its high hardness, thermal stability, and excellent nuclear radiation shielding performance, is a critical material for radiation shielding applications. It is widely used in aerospace, national defense, and medical fields. However, tungsten alloy has drawbacks such as high density and processing difficulty, which limit its applicability in high-efficiency, miniaturized, and lightweight designs, particularly in nuclear medical treatment and nuclear-powered flight devices. A double-layered W/Mg structure is expected to serve as a next-generation nuclear radiation shielding material. Due to the significant differences in melting temperatures and coefficients of thermal expansion between W and Mg, conventional soldering methods are ineffective for joining them. In this study, a W90 tungsten heavy alloy and AZ31B magnesium alloy were successfully bonded using an ultrasonic-assisted soldering method with pure Sn. The bonding temperature was 250 °C, and the ultrasonic duration was 4 s. The interfacial microstructure of the W90/Sn/Mg joint under varying ultrasonic power levels was analyzed using scanning electron microscopy and energy dispersive spectroscopy. Additionally, the shear strength of the joint was tested to assess the impact of ultrasonic power on interfacial bonding and mechanical performance. The results indicated that effective bonding formed at the W90/Sn interface, with Ni₃Sn₄ compounds appearing between the (Ni, Fe) matrix and Sn. The seam consisted of a β-Sn matrix phase and Mg₂Sn compounds, with an Mg₂Sn layer forming on Mg/Sn interface. As ultrasonic power increased, joint width decreased while the Mg₂Sn layer thickness increased. The shear strength of the joint initially increased with ultrasonic power but later decreased. At ultrasonic power levels of 100 and 150 W, the joint strength reached a maximum of 10.5 MPa, with failure occurring at the W90/Sn interface. When ultrasonic power increased to 200 W, joint strength declined, and fracture occurred at the Mg₂Sn layer. The acoustic pressure distribution of liquid Sn during the ultrasonic-assisted soldering process was simulated. During a single ultrasonic cycle, the acoustic pressure of liquid Sn oscillated periodically from negative to positive and back to negative values. The maximum acoustic pressure was observed at the center of the liquid Sn, gradually decreasing toward the edges. The cavitation effect, driven by collapsing bubbles, generated high temperatures, high pressures, micro-jets, and shock waves, as explained through theoretical calculations. Based on the Keller–Miksis equation, as ultrasonic power increased, the ratio of the cavitation bubble radius to its initial radius (R(t)/R₀) increased from 16.3 to 47.7, with collapse velocities ranging from 3262 to 6985 m/s. According to the Noltingk–Neppiras theory, as acoustic pressure increased, cavitation-induced temperatures rose from 14614 to 24989 K, while pressure increased from 1.08 × 10⁵ MPa to 9.45 × 10⁵ MPa. The generated temperature and pressure were sufficient to break the Mg alloy’s oxide film and promote interfacial reactions.