Abstract: By analyzing a tumor-immune model with exponential growth of tumor cells, the existence and stability of the equilibrium of the model are determined. The global dynamic behavior of the model is obtained by constructing Dulac function to exclude the existence of periodic solution. By means of numerical simulation, the dependence of dynamics of the model on the initial states and the possible phase regions including the non-tumorous region, tumorous region and cancer region are given. Again, the impact of the interaction coefficient between tumor cells and immune cells on the dynamic system of the model is discussed.