Image registration is an important research topic in the field of computer vision, in which the registration and mosaic of side-scan sonar images is the keypoints of underwater navigation. However, the image registration method of keypoints is not suitable for sonar images which do not have obvious feature points. Therefore, a method of sonar-image regis-tration and mosaic based on line segment extraction and triangle matching is proposed in this paper. Firstly, in order to extract features from sonar image, the LSD method is introduced to detect line feature from images, and line segments are filtered by the principle of attention; after that, triangles are formed from line segments, an image transformation matrix can be calculated through the heuristic greedy algorithm from these triangles; finally, images are merged based on the transformation information. On the basis of practical tests, it is found that, the feature extraction method used in this paper can better describe the outline of underwater terrain, and there is no obvious stitching gap between the result of sonar images stitched. Experimental results show that the proposed method is effective than the keypoints method of the registration and mosaic of sonar images.