To determine the corrosion wastage of the inner and outer walls of the old steel poles of the transmission lines, this paper explores the feasibility of ultrasonic guided wave B-scan detection technology. Firstly, by simulation analysis, the paper discovers that when the loss of corrosion thickness reaches 20%, there is an obvious stress concentration. Secondly, the model and theory of ultrasonic guided wave B-scan technology based on magnetostriction are introduced. By comparing the frequency dispersion curve of steel tube and polygonal steel tube, the propagation characteristics of guided wave in round steel tube and polygonal steel tube are analyzed, and the applicability of ultrasonic guided wave B-scan technology in steel tube detection is summarized. At last, the simulation examination shows that when using the highest sensitivity, the defects with the loss ratio greater than 0.86% of cross-section can be found, after through one weld, the defects with the loss ratio around 2% of cross-section can still be clearly identified. And it also proves the B-scan detection technology can meet the needs of inspection by field application.