Abstract: Appearance inspection, anatomical inspection, dye penetration test, infrared temperature measurement test was used to analyze the cause of a 500 kV line No.72 linear tower composite insulator broken series fault. The results show that the sheath of the faulty phase insulator has its own defects, lax seal and other issues, which leads to the contact between the mandrel and external air and rain, and the mandrel is corroded under the long-term action of strong electric field, acid solution and humid air, resulting in the breakage of the glass fiber in the composite insulator under the electrochemical reaction. The insulator eventually breaks the string under the combined action of wire weight and so on.