Focusing on the admission control problems of media delivery network in Software Defined Network (SDN), an admission control scheme of comprehensively optimizing the service performance of service nodes and transmission links was proposed. The proposed scheme used SDN controller's abilities of directly controlling routers and perception of entire network, to jointly optimize the service performance of the service nodes on the application layer and the transmission links between the service nodes and the users on the network layer, and the influences of link congestion to data transmission and quality of service were reduced. Firstly, the admission control process of the SDN service system was modeled as a Partially Observable Markov Decision Process (POMDP). Secondly, the observation-based randomized policy was used as the admission control policy of the system. Finally, the policy-gradient algorithm was used to optimize the admission control policy, and the optimal policy of the model was obtained. The simulation results show that, compared with the best effort service policy, the POMDP-based optimal admission control policy improves system performance by 10%, which illustrates the effectiveness of the proposed approach.