<p>To address the challenge of automatic weld seam tracking in submerged arc external welding of spiral welded pipes—where ultra-narrow or virtually non-existent grooves and severe surface reflections complicate the process—a novel low-frequency excitation electromagnetic sensor based on differential transformer principles has been designed. This sensor comprises three sets of coils and an E-shaped magnetic core. By detecting the differential variation in magnetic flux distribution between the two magnetic paths on either side of the core, this sensor converts the transverse deviation of the weld seam into a differential signal of the induced electromotive force (EMF) in the secondary coils, with the two exhibiting an approximately linear relationship. Key parameters affecting its performance were analyzed based on electromagnetic field theory. Through COMSOL numerical simulations and comparative experiments, the influence mechanisms of parameters such as excitation current, air gap height, and coil turn ratio on sensitivity were revealed, thereby determining the optimal design parameters for the sensor. In tests simulating extreme conditions of spiral welded pipes (ultra-narrow grooves, reflective surfaces), the sensor demonstrated good performance, achieving a tracking accuracy of ± 0.2–0.5&#xa0;mm, along with excellent robustness and test repeatability. This approach offers a highly reliable, low-cost technical solution to overcome the failure or stability limitations of existing laser vision, arc, and high-frequency eddy current sensors under extreme conditions. It lays the foundation for achieving high-quality automated welding in the external submerged arc welding of spiral welded pipes.</p>