An image fragile authentication watermark algorithm based on singular value decomposition and chaotic mapping was proposed, trying to apply chaotic mapping’s initial value sensitivity to tamper detection. Singular value decomposition was carried out on each image block, and authentication watermark was produced by using Logistic chaotic mapping. Then Logistic chaotic mapping was used to design a mapping function defined on two separate blocks, and authentication watermark sequence was embedded into the corresponding mapping block’s LSB. The experimental results show that the algorithm is not only very sensitive to tamper, but also resists collage attack and distinguishes image content tamper from watermark tamper.