We develop a queueing model to study the system performance under the most prevalent SYN-flooding attack, with reference different parameters and different situations, such as arrival rate, traffic rate, and timeout period, the maximum capacity of system, the standard deviation and the distribution of the service time. By simulation, we obtain some effective quantitative measurements (e.g. loss probability of regular request, utility for regular request or attack, and utility for whole server) and analyze the performances of the server under various conditions.