Abstract: Defects in external coating of EBV body results in CP current loss in long-distance pipeline and corrosion in pipeline adjacent to EBV is quickened. In the light of the problems, reason analysis on electrochemical corrosion is carried out. It is pointed out that if electrical isolation was installed adopting insulated joint and connected to trunk line with EBV, CP current loss would not take place. Solutions and technical links to be taken into account in design of EBV of trunk line are proposed.