In order to evaluate the reliability of a collector system of an offshore wind farm in the planning stage and to avoid maintenance difficulties caused by the poor accessibility, a method for calculating equivalent availability is proposed combining the probabilistic and deterministic methods. First, according to the characteristics of the largescale offshore collector system, it can be divided into a substation layer, turbine layer, and cable layer, and the availability of each layer of the ring collector system is described using the fault tree algorithm. Second, according to the possible operation state of the ring structure, the availability of a singlestring ring network is calculated. Finally, using a planning offshore wind farm as an example, the reliability and economy of the collector system are compared and analyzed, and different topological schemes of the collector system are also compared, providing quantitative reference indices for topological planning of the collector system of an offshore wind farm.