In recent years, the offshore wind energy sector has undergone a phase of accelerated advancement. In order to ensure the continued reliability of the transmission lines, enhancing the precision of transient overvoltage computations is of paramount importance. This paper presents a detailed analysis of the transient overvoltage mechanism of a 220 kV offshore wind farm and its delivery system. Additionally, an aggregation equivalent method for calculating transient overvoltage in offshore wind farms is proposed, taking into account the parameters that affect transient overvoltage in offshore wind farms. Verification of the method through PSCAD/EMTDC simulation demonstrated that it exhibits higher overvoltage calculation accuracy in the presence of asymmetrical ground faults and load shedding faults. This improvement in accuracy is 62.45% greater than that of the traditional single-machine multiplication method, with the maximum error remaining below 2%. The results demonstrate that the method is capable of more accurately reflecting the transient overvoltage fluctuations observed in offshore wind farms, thereby offering enhanced engineering application value.