Abstract: Reasonable maintenance of wind turbines is an important way to reduce wind farm operation and maintenance costs. Wind turbines in the same wind farm constitute a typical multi-component system,and the operating performance of each wind turbine determines the overall operating efficiency and maintenance requirements of the system. The maintenance effect of each wind turbine will also affect simultaneously the subsequent availability and maintenance decision-making of the system. In this paper,a conditionbased opportunistic maintenance strategy based on periodic detection under the imperfect maintenance is developed for the same type of multi-component system composed of main shafts of multiple wind turbines in a wind farm. A multi-state degradation space partition model is constructed to define the representation and relationship between the system state and the maintenance demand,besides,the calculation model of the system maintenance demand and the probability of state transition probability in the process of degradation and maintenance recovery are summarized and deduced. On this basis,an analytical model of system average cost rate is established to determine the optimal detection period and maintenance threshold. Numerical experiments are carried out to verify the correctness and effectiveness of the strategy and model through the actual operating data of the main shafts of a wind farm,and sensitivity analysis of the parameters is carried out to illustrate the applicability of the model. The results show that this strategy can effectively reduce the operation and maintenance costs of wind farms.