The topology and configuration of collector system is relative with security and reliability of wind farm, and the capital cost of the collector system constitutes a significant proportion of the total cost of the wind farm. Three main design basis of the collector system topology has been summarized, the reliability model and cost model has been respectively developed. The optimal design method based on K-means clustering and PSO algorithm has been proposed considering realistic engineering constrains. The test of the proposed method with a large offshore wind farm is given to show the effectiveness of the proposed method.