DC voltage divider is an important measurement equipment in converter station, the insulation failure of DC voltage divider will directly affect the operation of HVDC system. Two typical insulation faults of ±800 kV DC voltage divider are described, the disassembly status and insulation structure are discussed, and the potential and electric field distribution under different operating conditions of the voltage divider are calculated and analyzed, then the cause of faults is proposed as follows: under wicked external insulation operating environment, the insulation resistance of polymeric housed epoxy cylinder of the voltage divider nonlinearly declined, which makes the axial resistance of epoxy cylinder and resistance capacity units unsuitable, and further results in axial potential difference and causes partial discharge, creeping discharge of the epoxy cylinder, and radiate partial discharge and SF₆ gas-gap breakdown of the voltage divider. Based on the above analysis, the countermeasures are proposed such as improving the external insulation potential distribution, enhancing the threshold of partial discharge and breakdown, and carrying out the specific operation and maintenance. The research achievements will be of great significance to the selection and operation maintenance of DC voltage divider, and the improvement of operation reliability of ±800 kV DC voltage divider.