Abstract: With the DDoS (distributed denial of service) traffic which was implemented using depleted bandwidth is filtered by routers, the attached server owners should work together with ISP (Internet service provider) to defend against such DDoS attack the way the Netflow statistics is mainly used to allocate the weights for traffic routing by routers. A new algorithm is thus proposed to get efficiently maximum traffic by GA with the traffic filtered by routers, and its feasibility and validity have been verified in real network circumstances. The algorithm shows its advantages that it just occupies a small part of resources without any ISP router required to participate in, and the servers can be upgraded at low cast and allocated easily. Moreover, it can optimize the network traffic simultaneously with defending against DDoS attack, thus eliminating efficiently the global burst of traffic arising from normal traffic so as to improve greatly the efficiency of servers.