The standard of GB/T 28181 puts forward new requirements to the video security monitoring network system. This standard also provides specific process for SIP interaction and new expansion.The user agent is an important part of SIP equipment, SIP client and other entities. The user agent's main task is not only to meet the standard SIP processes, but also to serve as UAC to initiate a call and receive a call then respond it. The research and design of user agent has important influence on whether or not the video security monitoring network system is in line with the GB/T 28181 standard and system performances. This paper designs a standard and efficient user agent, which includes: the overall structure, key ideas of this design and how to construct SIP client based on user agent. In this paper, the design of user agent has many advantages,such as:highly modular, low coupling and high efficiency.The video security monitoring network system can easily and quickly build the appropriate SIP entity.