Reliability assessment of wind turbines can lay a foundation for its reliable operation. Using hydraulic system of wind turbine as an example, according to the system’s grey and complexity, a novel method is proposed by introducing the grey correlation analysis method into the fault tree analysis. This method adopts the dynamic identification coefficient principle instead of artificial setting 0.5, which expands the correlation distribution interval effectively. Also, factor weight coefficient is introduced to suffice the need of project. Application of this method shows that the importance of failure modes can be sorted and the weak link can be found. The results can be the theoretical basis of preventing accident and improving system reliability.