Abstract: Using a low order, two-layer baroclinic quasi-geostrophic model, a nonlinear system including the interaction between a thermal forced wave, a transient wave and zonal flow is studied. Under the conditions of near-resonance and weak baroclinic instability, the features of solution in the phase space are discussed with the analytical method of multiple scale and discontinuous oscillation. The results show that the dynamic coupling between the forces wave and transient wave is responsible for the physical mechanism of the nonuniform index cycle of the atmospheric circulation.