Abstract: In this paper, the coil spring of the core part of the operating mechanism in the high-voltage circuit breaker is taken as the research object. The dispersion curves and the relationship between frequency and incident angle of outer ring are obtained by numerical calculations. Based on the relationship, the ultrasonic guided wave probe for exciting specific guided mode is selected. The defect area containing two axial defects prefabricated at the different circumferential distance of the same cross section on the outer ring, as well as the defect-free area, are detected by the selected modes. The guided wave mode is determined by the end echo time and the acoustic distance. The distance between defect and excitation probe is calculated through defect echo time to locate defects.