Abstract: In order to realize the efficient detection of the internal and external strand fracture defects of the transmission line, this paper proposes a method based on ultrasonic guided wave to detect the internal and external strand fracture defects of the transmission line. Firstly, the numerical analysis method was used to obtain the ultrasonic guided wave mode and detection frequency suitable for transmission conductor detection. Secondly, simulation models for transmission conductors and defects were developed, with analysis conducted on longitudinal mode (L) propagation of ultrasonic guided waves, and the effect of ultrasonic guided waves on strand fracture defects in different materials was explored. The results show that the proportional coefficient established according to the frequency domain characteristics of the defect echo can distinguish the defects in the inner and outer layers, and the amplitude of the defect echo can reflect the degree of damage. Finally, experiments are used to verify the validity of the simulation results. The simulation and experimental results show that the L(0,1) ultrasonic guided wave method can quickly detect the broken strands of the transmission line, realize the positioning and quantitative analysis of the broken strands of the inner and outer layers, and improve the detection efficiency. This approach proposes an innovative methodology for broken strand defect detection in transmission conductors.