The purpose of this study is to establish a design method of tower service lift for offshore wind power system, as well as to conduct structure analysis of the service lift system. The service lift system will be built in the internal area of tower of the offshore wind power system. Design and structure analysis for the tower service lift system are conducted to clarify the stability and reliability of the system. Main objective of the design is to secure sufficient capability of transportation of workers and equipment with satisfactory performance within the designed tolerance limit. Total deformation and equivalent stress of the lift system by external load are examined using the results of structure analysis.