Abstract: In some industrial applications, a protective steel casing is added to the outside of the pipe for fluid transport, which makes it more difficult to detect corrosion of the inner pipe from the outside. Based on the pulsed eddy current testing technology, a flat u-shaped structure sensor is designed in this paper, which can detect the corrosion of the inner tube in the gap of the double-layer steel pipe without being affected by the casing. Moreover, this detection method can obtain the thinning information of the inner and outer wall of the inner tube through a single scan. In this paper, cylindrical probe and u-type probe were used to detect three defects of the specimen, and meanwhile, the gap between steel tubes was reduced to 25 mm and 35 mm, and the detection results were compared with those without casing. The results show that compared with the traditional coaxial cylindrical probe, the u-type sensor has stronger anti-interference ability to the external protection steel pipe, less influence on the attenuation curve, and higher sensitivity to detect the inner pipe between gaps, which can meet the demand of field detection.