Abstract: A method for the online detection of horizontal weld seam collapse was proposed, in which the weld seam collapse was detected according to the deformation of the 5×5 grid laser lines projected on the surface of the weld seam. Firstly, the images of the grid lines were processed and the vertical and horizontal pixel points were obtained. Then the vertical pixel points were fitted into five spline curves based on Gaussian function. Each of these spline curves was fitted into four segments, and the intersection points of the curve segment with the straight segment was obtained. The intersection points and the highest points on the five vertical spline curves were fitted into three straight horizontal lines based on a least square method. Finally, the collapse of weld seam was obtained by calculating the distances between the three straight horizontal lines. The experimental results show that this method realizes the online detection of horizontal weld seam collapse. This work is expected to provide the foundation for closed-loop control of horizontal weld seam formation.