The offshore DC wind farm with DC collection and transmission system eliminates the bulky power frequency transformer and multiple power converters. It has advantages on power density, cost and efficiency. These advantages let the pure DC system become an advisable choice of offshore wind farm. In the offshore DC wind farm, the high set-up ratio DC/DC converter is a very important part. This paper firstly analyzes the requirements of the step-up ratio, capacity and fault isolation for the DC/DC converter. Then a detailed classification is made about different kinds of DC/DC converters. The advantages and disadvantages of various DC/DC converter topologies and their prospects are pointed out. Finally, this paper compares the structure and control methods of these different topologies and affirms the future development of the high step-up ratio DC/DC converters which can be applied in the offshore DC wind farm. This work is supported by National Natural Science Foundation of China (No. 61374155), Research Fund for the Doctoral Program of Higher Education of China (No. 20130073110030) and Natural Science Foundation of Shanghai (No. 18ZR1418400).