Abstract: The transmission line tower would be corroded due to long time exposure in the atmospheric environment, which results in wall thickness reduction at different positions of angle steel sections and even leads to fracture and collapse. Therefore, the corrosion detection method for the inner wall of transmission line tower needs to be studied. Firstly, the propagation mechanism of ultrasonic Lamb wave is analyzed to determine the frequency detection mode of ultrasonic Lamb wave suitable for angle steel sections. Then, based on the finite element theory, the three-dimensional numerical model of angle steel section is established, the propagation characteristics of guided wave in steel material are explored, and the echo waveform characteristics at the end of angle steel section for different excitation frequencies are analyzed. Finally, the propagation characteristics of ultrasonic Lamb wave in the angle steel section under different defect conditions are obtained by numerical simulation for the angle steel section with corrosion defects at different positions and depths. The results show that the proposed detection method can effectively detect the defects of transmission line tower and provide a reference for practical applications.