With the slow start of offshore wind farm in China, the reliability of the offshore wind turbines needs emergency power supply with appropriate configuration under island operation mode as a guarantee, while offshore wind power’s island operation mode is facing many problems in equipment configuration selection, operation methods etc.. Therefore, this paper analyzes the necessity and applicability of the big island operation mode of offshore substation, and puts forward the island operation mode of the electrical system design scheme, and verify the applicable conditions of the scheme through simulation and analysis, which has been applied in two offshore wind farm projects and provides some reference to the offshore wind power project construction in the future.