In order to solve the problems of high maintenance cost of wind turbines and low efficiency of operation and maintenance resulting from unreasonable maintenance strategy, a research on maintenance strategies of a series connection system of wind turbines was carried out. The component fault was devided into potential fault and function fault based on the delay-time theory and a opportunistic maintenance strategy was proposed focusing on the preventive maintenance. A delay was made in the maintenance of components in search of maintenance opportunity. An opportunistic maintenance model for wind turbines was established with considerations of the influencing factors, like spare part logistics, downtime and maintenance costs. Using the field fault data, the Monte Carlo method was adopted to solve the proposed model and the influences of different inspection durations and different deferred maintenance time on the life-cycle availability and maintenance cost of wind turbines were explored. The results show that the opportunistic maintenance strategy considering deferred maintenance is superior to traditional preventive maintenance in cost-efficiency with engineering practice. With the optimal strategy obtained by the model proposed, the availability of wind turbine can rise up to 98%.