In order to optimize the topology of electrical collector system of offshore wind farm, the medium-voltage submarine cable cost, including investment cost and operating loss cost, which are both associated with the topology of collector system, is employed to establish the optimization model. FCM algorithm is improved to partition collector system, based on the minimum cable length. By adjusting the weights of each edge dynamically and iterating repeatedly on the Delaunay net generated on each subarea of collector system, the optimal tree topology with minimum total cost is obtained. Results of case study show that the optimization method is effective and the results of classical minimum spanning tree algorithm based on the distance between wind turbines are further improved. This work is supported by National Natural Science Foundation of China (No. 51507100).