Abstract: Aiming at the problems of complicated production process, high-cost and urgent need for copyright protection of forestry thematic maps, the invariant property of JPEG image compression was analyzed. On this basis, a DCT digital watermarking algorithm, in which watermark information was embedded in median and high frequency coefficients of forestry thematic maps using JPEG format, was proposed. The results showed that the algorithm had better imperceptibility and larger embedding capacity because the watermark information was embedded in the median and high frequency coefficients. Even if using lossy compression to encryption forestry thematic maps with quality factor that higher than preset value, the algorithm could exactly extract embedded watermark information, and had good robustness against noisy.