Abstract: Offshore wind farms obtain a fast development in China recent years,but relative technologies are always not perfect.For example,we are still lacking experiences on the design,unit type selection,construction,operation,and maintenance of offshore wind farms.To improve the evaluation system of offshore wind farms,this article puts forward a new index called ’theoretical completion rate of power output’ and gives its calculating method.Moreover,a case study is done to verify the feasibility.Our new index has two advantages based on the analysis.First,it represents the operation states of both turbines and the whole power plant at the same time.Second,it can combine wind resource and the running state of each wind turbine together.Thus,the index has strong operability and important reference value in evaluating the performance of offshore wind farms and improving the quality of operation and maintenance.