Abstract: The Gangneung and ± 500kV DC divider Longquan Converter Station external insulation flashover is analyzed. That the occurrence of flashover voltage divider reasons: First, hydrophobic composite casing completely lost; Second, the composite casing sheds less characteristic shape parameters; Third, the mean composite casing grading ring structure is irrational. Flashover occurs mainly due to hydrophobic composite casing completely lost, followed by grading ring structure is irrational. On this basis, for the voltage divider of the structure and nature of the incident,Propose a fundamental solution to the problem of thought is required by this article shall be electrical, mechanical and hydrophobic properties in the selection of composite insulation material casing; by down grading ring center distance and diameter of both grading ring composite casing ensures creepage distance and the effectiveness of dry-arc distance and the design value close to the main principles to re-design of grading ring ; the proposed umbrella out of composite casing greater than or equal 70mm, 70mm at least to re-pitch the umbrella design umbrella. Also suggests that the casing can be taken outside the insulation in the composite surface coating to enhance RTV-Ⅱ antifouling coating to increase the flash intensity of treatment measures of external insulation.