Aiming at the problem of poor quality of side-scan sonar image obtained by conventional image mending methods, this paper proposes a method of side-scan sonar image mosaic based on affine transformation method of triangulation. This method divides the strip image into a pair of triangles. While keeping the coordinates of one triangle unchanged, the other triangle is adjusted to fit with the adjacent strip image. Then affine transformation is used to realize the coordinate mapping for completing the seamless mosaic of the image. The experimental results show that:1) This method can effectively complete the seamless mosaic of images with a fine visual effect;and 2) It has higher image quality and better performance in terms of information entropy, signal to noise ratio and average gradient compared with gap filling method.