With the appearance of informatization period, digital watermarking is attached importance to a great extent in these facts such as copyright protecting, source authentication, private detection and on, and becomes one hot research field of information security. This paper proposes a blind watermarking algorithm based on B-rep for CAD models, the algorithm embeds watermarking aiming at NURBS surfaces or curves, and its merit is preserving the model’s shape exactly. Firstly, we go through the model for obtaining all NURBS primitives, and order these primitives by its length, then use interfacing knot insertion method to embed generated watermarking into ordered primitives. Semi-Blind watermarking is improved and becomes All-Blind watermarking, so we can directly extract watermarking from corresponding ordered NURBS curves, and don’t need provide origin model and origin watermarking messages. At last, through comparing the similarity both the extracted watermarking and origin messages to test if this watermarking is distorted or not. Results show blind algorithm can embed high-capacity watermarking into models, and reduce the time of extracting watermarking.