As an important part of the current collection line of the new energy station, the reliability of the three-core cable is very important. Simultaneous monitoring of leakage current at both ends of the cable can evaluate the cable insulation status online. The paper analyzes the factors affecting the double-ended distribution of leakage current in three-core cables. Theoretical and simulation analyses show that the cable grounding current includes sheath ring current and leakage current, with sheath ring current accounting for the main component; The magnitude of the leakage current at a defined voltage level depends on the cable length, and the double-ended distribution of the leakage current can be influenced by the impedance characteristics of the cable shield and the grounding body. The research in this paper provides a theoretical basis for the online assessment method of cable insulation condition based on the synchronized monitoring of double-ended grounding current.