Abstract: An analysis was conducted on the causes of fracture and detachment of cast aluminum tail tube in 110 kV cable terminal, and the influence of porosity defect on overall performance of the tail tube was investigated. Through technical methods such as chemical composition analysis, mechanical performance testing, and scanning electron microscopy, accident causes were discussed from multiple perspectives, considering factors such as material, structure, and operating conditions. Numerous pore-type defects were identified in the faulty tail tube, along with material composition segregation. Results indicated that the fracture occurred at transition corner of the tail tube flange, which was the location with the largest cross-sectional change in the connection between cable terminal and gas insulated switchgear (GIS). Micro-cracks initiated at the edges of pore defects due to notch effect and propagated under alternating loads such as those induced by GIS electromagnetic force. Moreover, inhomogeneous material composition led to an overall decline in comprehensive mechanical properties. During long-term operation, cracks first developed at tail tube corner, where stress was most concentrated, resulting in insulation breakdown of the tail tube. Under the impact of arc voltage accompanied by sustained short-duration fault current, a overall brittle fracture ultimately occurred, leading to detachment.