Abstract: This study aims to investigate the fatigue life and failure mechanism of HIC encapsulation shells based on parallel seam welding technology under random vibration load, providing theoretical support for the fatigue design and optimization of encapsulation shells. First, encapsulation shells were prepared by welding 10# steel bases with 4J42 alloy cover plates by using parallel seam welding technology. The welding quality was verified through the weld’s microstructure analysis, revealing uniform structures, smooth transitions, and no obvious macroscopic defects in the weld zone. Subsequently, random vibration tests were conducted, and failure modes were determined through fracture analysis. The results show that when the random vibration loading direction is perpendicular to the cover plate, and the vibration level reaches 25 g, fatigue failure occurs in the inner weld zone at the middle of the long edge of the cover plate, with cracks initiating at the weld edge and propagating along the thickness direction of the cover plate. Additionally, sinusoidal vibration tests are performed, and finite element analysis is conducted on encapsulation shells by using ABAQUS to calculate the stress response at critical locations. An S-N curve is constructed based on the failure times and stress data. Finally, frequency-domain data of stress under random vibration is transformed into time-domain data using the inverse Fourier transform, and the fatigue life is predicted by using the rain-flow counting method and linear damage accumulation method. The results indicate that the fatigue life of the encapsulation shell under a 20 g random vibration load is approximately 37.69 hours.