By the low-order spectral model, the abrupt change of the circulation in the barotropic atmosphere is studied. In certain parametric combinations, along with the gradual change of the external thermal forcing, the quasi-cusp pattern abrupt changes of both the spectral coefficients and the locations of west jet stream are stimulated, which are similar to the rapid northward jumping and suddenly establishing of the “South-Branch” jet stream during June and October respectively. Depending on the changes of the parametric combinations, the motional form of the abrupt change can be substituted by that of the gradual change of one stable equilibrium, or by that of the branching phenomena of the multiple equilibria. The abrupt change mechanism of the atmospheric circulation is a nonlinear mechanism which contains the thermal forcing, dissipation, and thermal and dynamic effects. By the low-order spectral model, the abrupt change of the current in the barotropic atmosphere is studied. In certain parametric combinations, along with the gradual change of the external thermal forcing, the quasi-cusp pattern abrupt changes of both the spectral coefficients and the locations of west jet streams are stimulated, which are similar to the rapid northward jumping and suddenly establishing of the “South-Branch ” jet stream during June and October respectively. Depending on the changes of the parametric combinations, the motional form of the abrupt change can be substituted by that of the gradual change of one stable equilibrium, or by that of the branching phenomena of the multiple equilibria. The abrupt change mechanism of the atmospheric circulation which includes the thermal forcing, dissipation, and thermal and dynamic effects.