In recent years, with the continuous advancement of multimedia technology and network technology, network-based multimedia applications are increasing day by day, both in scale and in the number of users. However, the randomness, time-varying, and complexity of user behavior and network make it impossible to guarantee the service quality (bandwidth, delay, etc.) of multimedia services, thus making it difficult to meet the demands of growing users for multimedia services. While meeting the application requirements, how to design and develop a scalable multimedia service system has become one of the key issues to support good multimedia services and better develop multimedia service services. In addition, the surge in user demand comes with various challenges. High-definition and smooth multimedia services require stable bandwidth. However, the randomness, time-varying, and complexity of user behaviors and network states usually make it difficult to guarantee the quality of services. It is of practical significance to study how to overcome these problems, and it is also a hotspot of current research. This paper mainly discusses the extensibility elements of the multimedia service system and puts forward the relationship between the extensibility of various elements and some design principles of the extensibility. In order to overcome the influence of random variation of the wireless channel on video playback, this paper adopts scalable coding technology. That is, through the analysis of the playback process, the underflow probability is proposed and defined, thereby establishing a constrained optimization problem. Finally, this paper also proposes the multimedia adaptive transmission in the network to do the optimization.