In the process of high speed gas metal arc welding (GMAW), large box girder has many complex working conditions, such as positioning weld, low assembly accuracy, difficulty in strictly ensuring the pose of workpiece through tooling, real-time change of 3D pose of weld, etc. The vision based weld recognition method has a large amount of calculation and is not aimed at the workpiece with positioning weld, making it difficult to obtain the 3D pose of large box girders quickly. Aimed at this problem, a fast 3D pose estimation method for large box girder based on laser displacement sensing and point cloud clustering was proposed. Using this method, the vertical plane and flat plane of the welding seam of large box girders were obtained by fast segmentation of point cloud. Then, the pose information of welding seam was calculated. A pose information estimation test was conducted for welds with different poses. The results show that when the welding speed is up to 1200 mm/min, the pose error of the weld is less than 0.25 mm and 1.8°respectively. The robustness of the automatic welding of large box girders to complex conditions of positioning weld and low assembly accuracy is enhanced, which greatly improves the welding quality.