Image watermarking resisting to geometric attacks is challenging at present. As typical geometric attacks, affine transformations can defeat most of existed watermarking schemes. Based on watermarking model with side information, a watermarking algorithm is proposed in this paper which is used of zernike moments of cover image. Employing dither modulation，watermark is coded and embedded by taking the zernike moments of the normalized image in the way of side information. It has been proved by experiments that this algorithm is high imperceptibility, high capacity, and robust to affine transformation and JPEG lossy compression.