Abstract: To keep algorithm's high robustness and accuracy,a semi-fragile digital watermarking algorithm based on slant transform was proposed.The algorithm introduced the conception of "relevant-block-group",finished the process of watermark embedding by utilizing the characteristic of high energy concentration and the principle of the DC coefficient quantization,and used positioning reference blocks during the process of authentication.Experiment results show that,to some extent,the algorithm tolerates the accidental tampering,such as the JPEG compression,gauss noise,etc.The tampered areas can be detected effectively and located accurately.