A dynamic network security system and a control method thereof in a router where an Intrusion Detection System (IDS) and a Voice over Internet Protocol Application Level Gateway (VoIP ALG) are integrated, system including: a VoIP ALG module for acquiring VoIP IP/port information of a counterpart unit in use for determining whether or not to perform intrusion detection on a packet received via VoIP signaling with the counterpart unit; an intrusion detection module for comparing the received packet with a preset intrusion detection log entry to perform intrusion detection on the received packet, and based on a result of the intrusion detection, determining whether or not to allow passage of the received packet; and an IP/port check module for checking VoIP IP/port information of the received packet according to the VoIP IP/port information of the counterpart unit provided from the VoIP ALG module to determine whether or not to perform the intrusion detection, and providing result information on the determination whether or not to perform the intrusion detection to the intrusion detection module.