Abstract: A new type of seam-tracking sensor was designed based on alternative current field measurement in order to provide effective tracking of narrow-gap I-type groove butt weld.A 3D simplified model of magnetic field was developed to simulate the generation of inductive current on the weld surface by the sensor, the calculating formula of spatial magnetic induction intensity was derived, and the 3D distribution of magnetic induction intensity was simulated using MATLAB software.The relationship between the weld position and magnetic distortion signal caused by weld seam was analyzed, the mathematical relationship between the magnetic induction intensity of weld seam and deviation distance around weld seam was established with the approximate two polynomial curve fitting, and a new method to detect the deviation of weld seam was put forward.