Digital image watermarking is a kind of information hiding technology. Due to high demand of secretiveness and reliable transmission for the secret message in information hiding technology ,the paper adopts the unequal error protection of LDPC coding modulation (LDPC-CM-UEP) scheme to apply to digital image watermarking technology, the code words which the watermark information is encoded by LDPC code to generate are divided into two parts.The two parts respectively make use of two different QAM high-efficient modulation modes, so that the watermark information as important bit gets very strong protection. The simulation results show that this scheme improves the spectrum utilization rate of the system on the premise of guaranteeing robustness and imperceptibility of the watermark, thus improving the transmission quality of digital image watermarking system.