Abstract: DoS (Denial of Service) attacks have become one of the most severe security threats to the global Internet increasingly. It makes the resources of Internet used up and incurs congestion by sending a large number of packages constantly. This paper proposes using signal processing method to analyze DoS attacks. Because of sending mass packages, it must have much more power. For this, the FIR filter is designed to filtering the illegitimate frequencies in frequency domain and the LAR (Legitimate traffic to Attacked traffic Ratio) is improved. Then much more resources and service can be supplied for the users.