The operation and maintenance cost of wind farms accounts for about 1/3 of its revenue. The optimal maintenance of wind turbines is always the main way to reduce operation and maintenance cost of wind farms. Focusing on a system composed of multiple identical wind turbines, this paper presents a condition-based opportunity maintenance strategy for it. A deterioration state space partition method is proposed to analyze and model the opportunity maintenance strategy for the multi-unit system. Then, an analytic model is established based on the presented modeling method to determine the optimal inspection period of the system, and opportunistic and preventive maintenance thresholds of each unit jointly for minimizing the long-term cost rate. The experimental result verifies the performance of the modeling method. It also shows that the joint optimal maintenance decision can achieve significant performance for the wind turbine.