Abstract: In this paper, the physical factors related to the formation of the equivalent barotropicity of atmospheric circulation anomalies are studied using a two-layer linear quasi-geostrophical model. It is pointed out that the horizontal scales of disturbances, the β-effect and the thermal wind which is the vertical gradient of the mean westly wind are three major factors in maintaining the atmospheric equivalent barotropicity. The in-phase relationship between the stream function and the thickness field in the mid-troposphere resulted from the equivalent barotropicity is also discussed. At the end of this paper. the deficiencies of the linear model are pointed out.