Public security system is a special system that should aim at high speed, reliableness and comprehensiveness, fully integrated by video conferencing, data collaboration, multi-channel video surveillance and other operations. As a result, the public security system design must take full account of the industry users' individual character and special function requirements. In this paper, based on the existing network security system and SIP protocol, a video conferencing system is designed and implemented, which can help SIP further study and application in public security system effectivelly.