This paper discusses the redundancy issues of the collection and transmission systems of large-scale offshore wind farms. Firstly addresses the importance of wind farm reliability study; then from the analysis of existing wind farm design researches, quantitively defines the redundancy degree of wind farm collection and transmission system. After analysing the wind farm geographic features, proposes a systematic wind farm redundancy design method, considering both reliability and economic costs. An example offshore wind farm design is carried out with the proposed method and gives optimal design results. The proposed redundancy design method proofs its advantages in complex large-scale offshore wind farm collection and transmission system design.