This paper briefly analyses the type of service of the GBS(Global Broadcast Service),put forward an approach of the service modeling based on Hidden Markov Model(HMM). By simulating the GBS in Qualnet, the service traffic trace is captured. The proposed modeling approach is realized in Matlab. The HMM based modeling approach only used two variables: packet size and inter packet time, which are hardly influenced by encryption techniques. The experiment results show that by the use of discrete variables, the proposed modeling approach can decrease the calculating complexity and model constructing time, improve the time cost.