The operation and maintenance cost is a significant portion of the entire lifecycle cost of offshore wind farms, therefore reducing the operation and maintenance cost is the key to ensuring the economic benefits of offshore wind power. The current operation and maintenance methods for offshore wind farms are analyzed and the operation and maintenance plans comprised of various Crew Transfer Vessel (CTV), Service Operation Vessel (SOV) and helicopter are designed. Based on the wind farm location, operation and maintenance method, operation and maintenance time and operation and maintenance plan, a model is presented for minimizing operation and maintenance cost and the optimal number of operation and maintenance teams for kinds of plans is obtained. Taken a Jiangsu wind farm as an example, based on the characteristics of the wind farm, the actual operation and maintenance plan is designed to obtain the best operation and maintenance plan.