In order to ensure the safe and stable operation of the power system, the VPN network border security defense system of power enterprises is designed. The hardware of the system is planned and designed, including cooperative intrusion detection, security audit, cooperative camouflage, collaborative firewall, disaster recovery and electronic forensics. On the basis of the hardware design, the network data in the gateway is detected by using the cooperative control framework and other security technologies, and the attack behavior is analyzed by using host based intrusion detection, and the network information security situation assessment based on si-s0 is proposed. The algorithm estimates the security state of network intrusion information and realizes the security defense of network boundary. The experimental results show that the designed VPN border security defense system has higher security than the traditional system, and fully meets the research requirements.