Based on the arc spectral radiation characteristics of ZL201A aluminum ally in CMT + P process and camera’s quantum efficiency, we select 520 nm band-stop as the working band for weld pool imaging acquisition systems and obtain high signal-to-noise ratio aluminum alloy weld pool images at the base level of weld current. Through analyzing the characteristics of captured aluminum alloy weld pool image, a semi-supervised and adaptive contour extraction method is designed, called Maximal Similarity based Region Merging and Adaptive Matting (MSRM-AM). Firstly, an interactive image segmentation method based on super-pixel is used to obtain the initial contour, and then the initial contour is finely segmented to get the final segmentation result. Comparing with other contour extraction algorithms, the proposed algorithm has higher accuracy and reliability, and has verified through abundant experiments.