Abstract: The structure of transmission line steel pipe is a kind of large-diameter thin wall tube with the taper slope When the ultrasonic guide wave excited by the circumferential sweeper propagates in the steel pipe, the phenomenon of beam diffusion occurs, which causes circumferential resolution reduction in ultrasonic waveguide detection of steel pipe. In order to solve the problem, a method of synthetic aperture focusing technique (SAFT) in frequency domain is proposed for the ultrasonic waveguide imaging inspection of steel pipe. The inspection device constructed in this paper is used to conduct ultrasonic guided-wave scanning of a steel pipe with a diameter of 608 mm to obtain B-scan images and frequency-domain SAFT images. Comparisons show that the circumferential resolution of the scatterer and through-hole in the frequency domain SAFT image is obviously better than that in the B-scan image, which indicates that the SAFT in frequency domain method proposed in this paper can significantly improve the circumferential resolution of the ultrasound guided wave in detecting steel pipe.