Due to the rapid spread of botnets, distributed denial of service attacks have become more and more frequent and fatal. In order to detect and defend DDoS attack, many researchers have been done, such as using NS2 to simulate such attacks. However, the major bottlenecks in large-scale network simulation are the memory usage and simulation time. In this paper, the authors present a novel approach to simulate DDoS attack, called Analytical Packet Forwarding Model (APFM), which simplifies the procedure of packet forwarding based on computation. The experimental results show that the model ensures the authenticity of the overall simulation procedure and simulation results, compared to NS2.