Abstract: Overvoltage level is the key parameter of the offshore wind power transmission system, which has significant impact on the system design and equipment selection. In order to evaluate the key impact factors of the overvoltage level of the offshore wind farm transmission system and calculate its overvoltage accurately, the overvoltage mechanism of offshore wind farm 66 kV submarine cable transmission system is studied in this paper. Firstly, the topology, control and protection strategy of the 66 kV offshore wind power transmission system are analyzed. And then its overvoltage mechanism is studied from the aspects of fault-free load shedding overvoltage, single phase to ground fault load shedding overvoltage and load-free overvoltage. On this basis, the influence of the neutral point grounding resistance on the overvoltage of the 66 kV transmission system is studied. Finally, a simulation model of the offshore wind power transmission system is established in the PSCAD/EMTDC environment and simulation studies are conducted. The research results show that the grounding resistance of the system and the control and protection strategy of the wind turbine are the main impact factors of the overvoltage level in the 66 kV transmission system. With the variation of the grounding resistance, the control and protection actions of the system will change. The system shows different overvoltage characteristics under different grounding resistance ranges.