Abstract: Feature points of the weld profile are the guide for bead planning and seam tracking in robotic welding of thick steel plates. In this paper, the extracted weld profile of T-joints was studied, and a method to extract feature points of the weld profile of T-joints was proposed based on span segmentation of slope monotone intervals using Otsu. Firstly, slopes are calculated effectively using the preprocessed weld profile data, and followed by smooth processing. Then these intervals whose slope spans are bigger than the average one are selected. In order to determine the final slope monotone intervals from the selected ones, the sloe span values are segmented by Otsu, the centers of the intervals can be deemed as the feature points of the weld profile. The advantages of the method are that it can run well without any empirical parameters and can be applied to the typical welded joints.