The study of condition-based maintenance strategy for the gearbox is crucial to reduce maintenance costs and improve reliability of equipment since its maintenance costs is one of the highest parts in wind turbine. In allusion to imperfect maintenance of the gearbox, an optimized maintenance strategy based on proportional intensity model (PIM) is proposed. The strategy builds the PIM based on the monitoring data about gearbox vibration, temperature and historical maintenance, and obtains the intensity function of gearbox; then the physical programming method, which is applied to deal with the tradeoff between the minimizing maintenance costs and maximizing reliability, is used to determine the optimized maintenance threshold and formulate the optimal maintenance strategy. Finally, the optimized maintenance strategy with imperfect repair is simulated by using the actual failure data and online monitoring data from wind power plant. The simulated results verify the effectiveness and rationality of the proposed maintenance strategy for gearbox.