Regardless of many researches done in recent years, most wind turbines are still unable to reach their design lifetime [5]. Failures in the gearbox, especially in the planetary stage, have been a major cause of reliability problems in the modern wind energy turbine system. The following paper proposes a fault diagnosis method based on the strain signal of the ring gear. First, the strain signal is collected from the side of the ring gear using FBG sensors in normal condition and faulty condition. Then the collected strain signal is processed and analyzed. In the time-domain analysis, traditional statistical indicators like Peak to Peak, Kurtosis, Crest factor and Peak value are adopted. The analysis results show the effectiveness of the proposed method for identifying tooth crack fault of the ring gear.