Abstract: The complex dynamic behaviors, especially the behavior of transition between different states, of a closed reaction system which involves only bimolecular reactionsteps (A+B→C+D, B+C→2C) and unimolecular ones (B→P, E→A, G→B, A→D,C→P), is studied. By using the quasi-steady state analysis, a quasi-steady state bifurcation diagram, which predicts the existence of bistability between two different steady states or between a steady state and an oscillatory one, is obtained and confirmed by numerioal simulation. The numerical simuiations show that various transition transitions are trigged by external perturbations, and some may occur spontaneously. The numerical simulations also show that the system may exhibit the phenomena of excitation. All these phenomena are similar to those observed exprimentally in realistic closed reaction systems.