Abstract: A case of 220 kV marine cable breakdown was analyzed, which is thought caused by local overheating fault of sheath grounding structure. Based on the ATP-EMTP software, the current distribution of metal sheath and armor structure under different grounding contact resistant was studied. The results show that the grounding current distribution is mainly affected by the ratio of sheath/armor grounding circuit resistance. Based on the field practice, a simulation model of grounding defects is built, which verifies that the improper grounding of armor and the damage of semi-conductive layer are the main reasons for the increase of sheath grounding current.