Abstract: Through a two-level low-order quasi-geostropic model,it is learned that equivalent barotropic state is a equilibrium state of the model.The existing conditions of the equivalent barotropic Rossby wave is discussed and then an oscillation equation reflecting the variation of the phase difference in low and up-level with the stationary Rossby wave (equvalent barotropic structure) as basic state is derived.The results show that there is relation among the variation of phase in low and up-level,the disturbance of vertical shear flow,the disturbance of Phase of the stream and that of thermal wind in middle-level.Finally,it is found that the eouivalent barotrooic and baroclinic Rossbv waves can be transformed to each other.