Unlike weld seam detection in a welding process, weld line localization for inspection is usually performed outdoors and challenged by noise and variation of illumination intensity. In this paper, we propose a weld line localization approach for mobile platform via a cross structured light (CSL) device and spatial-temporal cascaded hidden Markov models (HMMs). A CSL device is designed to project cross red laser stripes on weldment surfaces and capture the weld convexity in video sequences. Stripe edge images are extracted and then a spatial HMM is designed to detect the regions of interest (ROIs) in the video frames. Detected ROIs in successive video frames are fed to the proposed temporal HMM as observations to track the weld lines. In this way, we incorporate both the spatial characteristics of laser stripes and the continuity of the weld lines in an optimal framework. Experiments show that the proposed approach can effectively reduce the influence of illumination and noise, contributing a robust weld line detection and tracking system.