A modified susceptible-infected (SI) model is developed in this paper. The mean field approximation method is further adopted. This present research is based on traffic-driven epidemic spreading behavior. Take MIP routing for example, diffusion coefficient τ, traffic generation rate λ, infection rate β and their relationship with the actual betweenness bk are re-explored under the SI model. Both the theoretical and experimental results indicate that under the same internet topology and routing strategy, the diffusion coefficient τ is inversely proportional to average traffic generation rate λ and infection rate β.