An improved digital watermarking algorithm based on Block-SVD is put forward, which has better robustness. It can deal with the rectangle matrices directly and can extract better-quality watermarks. It takes little time to embed and extract the watermark in large images and this method can avoid some disadvantages such as the distortion caused by the computing error when extracting the watermark in the diagonal direction. Robustness testing results and time comparing analysis between SVD and Block-SVD are presented in this paper. Compared with traditional Cox method, this algorithm has better robustness, faster speed and is more practical.