Under long-term environmental impact of electromechanical load, pollution, ambient temperature and humidity, the micropore and crack in cap and pin of porcelain Insulators will gradually expand, the mechanical strength and insulation performance deteriorate. When subjected to lightning stroke or pollution flashover, it is likely to cause vicious accidents such as explosion or insulator string fracture. In this paper, lightning stroke-caused fault of 500 kV line porcelain insulators is analyzed, combined with lightning positioning system, line distributed fault location system and insulators' test. It can conclude that a number of low (zero) resistance insulators in the string exist before lightning flashover, when the shielding failure lightning strike C phase and causing the insulator string flashover, most of the power frequency freewheeling energy is discharged from the low (zero) value insulator in the porcelain discharge channel, and the heat generated causes the insulator to explode, causing string fracture. It is recommended to check the low (zero) value insulators in the line, adopting leading-type lightening protection measures such as installing line surge arrester or coupling ground wire in heavy lightening area.