A quadratic watermark theoretic model based on image content was proposed. The model extracted MD5 value firstly, which is the unique identification of information abstract of the origin image. Then the MD5 value was embedded into the origin image together with image watermark, which makes the embedded watermark include both contents relevant and irrelevant to the content of the origin image. The robustness of the watermark system was realized by using image watermark of practical content; the explanation attack was resisted with the association of MD5 value and the origin image. Experiments results showed that the method proposed is simple, feasible, low embedded data flow, high relevancy to the image file, and high robustness.