Abstract: In order to clarify the characteristics and level of the overvoltage of offshore wind power AC transmission system, the fault ride through characteristics of wind generator should be considered. Based on the control principle of the converter of permanent magnet synchronous generator, an analytical method for calculating the asymmetrical grounding overvoltage of offshore wind power system is proposed. Based on PSCAD, precise electromagnetic transient modeling and fault voltage ride through test are carried out for the wind generator model. On the background of a large-scale 500kV offshore wind power project in China, asymmetrical grounding and load rejection overvoltage of the system are simulated and calculated. The results show that the magnitude of asymmetrical grounding overvoltage in the system is related to the positive and negative sequence control strategy of the wind generator. The transient overvoltage above 1.3p.u. lasting for hundred-millisecond will be generated during the load rejection at grid side of submarine cable. The longer the overvoltage protection action delay, the more serious the transient overvoltage. The actual calculation of offshore wind power engineering needs to be closely related to the wind generator control and protection strategy, and evaluated based on the voltage tolerance level of key equipment. By optimizing the wind turbine control strategy, the system overvoltage level can be reduced.