Abstract: Debonding and cracking encapsulant for a bus plug appeared during assembly. The causes of debonding and cracking of the encapsulant were analyzed by means of fracture analysis, mechanical property test, ground test, air test and so on. The results show that the mechanical properties of the silicone rubber used in the encapsulant was not good enough, and debonding and cracking of the encapsulant happened under the bending stress during bus assembly. A kind of silicone rubber with high elongation was selected through experiments, which can be used with base coating process to solve the quality problems of debonding and cracking of the encapsulant of the bus plug.