For the purpose of image tampering localization, a fragile digital watermarking method is proposed on the basis of orthogonal fractal coding and chaos. The algorithm use the fractal coding parameters as a key to create the chaotic binary watermark to be embedded into the fractal coding field, so as to realize the image authentication. The algorithm can effectively detect whether the image is malicious tampered, and different host image will produce different chaotic binary watermark, which strengthen the confidentiality of the algorithm, especially in the face of mass image processing. The simulation results show that the proposed algorithm is efficacious and practical, and has precise tamper localization to grayscale images.