The seal at the joint of the composite insulator end fitting and the mandrel sheath has not received sufficient attention, which may be the cause of brittle fracture of the mandrel and interface breakdown. The end connection structure of the silicone rubber insulator is mostly wedge-shaped, and slippage between the mandrel and the fitting during long-term heavy load operation is unavoidable. When the sealant and the fitting have poor adhesion, the slight slip is enough to make the seal open; when the sealant is not elastic enough, the excessive slip can break the sealant. From this perspective, the use of a crimped end structure is one of the ways to prevent seal damage. In addition, the engineering application sealant is mostly room temperature silicone rubber, its anti-aging performance is not as good as high temperature silicone rubber, which may lead to premature deterioration in the case of corona in the end fittings. For products of 110 kV and below, it is more dangerous to not use the pressure equalizing ring, or 220 kV and the upper pressure ring of the upper product. In short, whether the end seal is maintained for a long time under long-term electromechanical action requires trial demonstration and long-term operation.