The one of the important reason which lead to power line dropped is interface aging. According to the core fracture of composite insulator on the 500kV power line of CSPG and the temperature abnormal rise of composite insulators which number are 25, 38, and 42 on No. 5123 tower XuYang power line of Shanghai power company of SGCC, these composite insulators had been done test about each appearance. The results of test show that the hydrophobicity is well and the material of silicon shed isn't aging. But there are some humps on the surface of housing and erosion on the interface between whole housing and core of composite insulators. The penetrant (1%fushine) appear on the interface between housing and core. The erosion had extended from high voltage to the middle part of composite insulator. One of the reasons about erosion on the interface between Housing and core of composite insulators is mold releasing agent seeping from core. The crack between housing and core has been made by the mold releasing agent and the partial discharge has appeared. The water which were absorbed on the production of core has taken part in electrochemical reaction lead to erosion.