At present, VSC-HVDC transmission is a common choice for offshore wind power grid connection. In the whole wind power transmission system, DC submarine cable, as the connection unit of VSC-HVDC converter stations, plays a crucial role in the safety and operation reliability of the whole system. Based on the DC submarine cable in a planning offshore wind power VSC-HVDC transmission project, a simulation model of the whole offshore wind power VSC-HVDC system is established in PSCAD/EMTDC. A total of 17 kinds of fault types may happen in the system are considered. The fault zones include coupling transformer grid side, coupling transformer valve side, bridge arm reactor valve side and DC side. As a conclusion, the highest core-ground overvoltage of ±250 kV submarine cable is 506.4 kV and the corresponding fault type is the grounding fault at the start point of DC submarine cable. Considering 30% insulation margin, the insulation level of the DC submarine cable with rated voltage of ±250 kV is determined to be 660 kV. The calculated results can be an important reference for improving the safety of submarine cable operation.