As the DDoS (Distributed Denial of Service) attack becomes more and more popular and the IDS evasion technology continuously renews, the security of the Internet attracts more attention of researchers and attackers. However, the evasion technology against IDS needs a lot of advanced work and has strict requirements for attackers. Thus, the attack to the IDS is paid more attention to because of attacking the targeted host easily. Among different attacks, DDoS attack implements relatively easily, and one typical popular attack frequently seen in large scale DDoS attacks is the reflection amplification attack with a little difference with the DRDOS (Distributed Reflection Denial of Service). To this end, we provide a detection method for this attack specially. And the result of the experiments shows the method is of great theoretical and practical value.