In this paper,a multi-level and dynamic security access control(MLDAC)model is proposed for CAD models in collaborative environment.A multi-level privilege model is developed to simplify the process of permission definition and assignment for enriching the expression ability and helping to realize multi-grained access control.The dependent relation of permission and the permission state migration are brought into MLDAC for dynamic authorization management based on the basic theory of workflow.Based on the practice,MLDAC model is more efficient to control collaborative operations.It meets the characters of design tasks,i.e.divisible,dependent and interactive.