With the rapid development of computer network technology, the security of computer network becomes increasingly important. Three main threats facing computer network security include: hackers, computer virus and denial of service attack. Things leading to the safety of the network are mainly: resources sharing, data communication, computer virus and TCP/IP protocol security flaws. A safety network system should include at least three kinds of measures: legal measures, technical measures and review and management measures. The paper analyzes the main threat facing computer network security, discusses network security technology and advances some effective countermeasures in view of the hidden danger of current common network security.