The digital watermarking methods for packing and decoration drawings were primarily aimed at process color, which had poor performance in spot color printing. Based on the spot color printing and combined with Integer Wavelet Transform (IWT) and Singular Value Decomposition (SVD), a robust watermarking algorithm for spot color printing was presented herein. Firstly, spot-channel image was transformed with IWT, and the watermark was then embedded in singular values of the 1-level IWT decomposed sub-bands. To improve the security, Arnold scrambling and encryption was adopted and a digital signature was embedded in the watermarked images. Based on the features of print-scanned image, the spot color printing image was scanned into grey image in extracting process, and the extracting digital signature was required prior to the extraction of watermark information. To verify the proposed method, the offset printing was chosen. A series of tests were conducted and compared with the method in paper [8]. The results showed that the method was effective to resist the attack from offset printing, and the imperceptibility, capacity and security in the spot color were also favorable.