Abstract: As SPKI technology has advantage on the aspect of authorization, this paper presents an access control system for large multicast system named MDAC based on SPKI and MDAC authentication and authorization protocol was also designed. Experiment of simulation and performance evaluation was also done. Compared with other solutions, MDAC has not only excellent performance, but also characteristics of distribution, supporting asymmetrical multicast, delegation and privacy protection, less computing cost in border router.