Abstract: An investigation is done on an explosion accident of 500 kV oil-immersed current transformer.Through diagnostic test and disassembly analysis, it was comprehensively determined that the main cause of the accident was that the insulation level of the current transformer was too low, the insulation deteriorates during long-term operation and comes to the breakdown of the main insulation by time. The corresponding measures are put forward for the accident, which offers reference for fault diagnosis, operation and maintenance of such current transformers.