Based on the present situation of CAD document protection system, the paper focuses on the innovation of CAD security model. It analyses current problems and puts forward a CAD model protection algorithm whose basis is a multi-level and dynamic permission strategy. This algorithm allows different access permissions or operations to the CAD models while in different phases of the product life cycle. Combined with the technologies of Access Control, Permission Manage, Encryption and Decryption, the paper successfully makes the security control of the CAD model more reliable in its product life cycle.