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 researches 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, we 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 our model ensures the authenticity of the overall simulation procedure and simulation results, compared to NS2. The most significant contribution of APFM is that it significantly reduced the memory usage and simulation time.