Aiming at the problem of real-time weld position detection in seam tracking, a line laser detecting sensor which could abstract 3D data of the seam contour was designed. First, an industrial camera with filter collected image of the laser stripe which was projected on the weld surface by a laser with specific wavelength. Second, the center line of the laser stripe was extracted after image enhancing, binarization, noise filtering and thinning through algorithms of image processing. Third, based on the triangulation method, the 3D coordinates of seam contour were calculated from the pixel coordinates of the center line. At last, two detecting experiments were carried to verify the measurement precision and anti-interference ability respectively. The experimental results indicate that measurement precision can reach ±0.2 mm with good anti-interference ability to arc and spatter, which can satisfy the requirement of welding.