Abstract: Combining weak nonlinear theory and dynamic divergence theory, the physical mechanism of discontinuous periodic oscillation of high and low exponential circulation is analyzed. The results show that: When the vertical shear is large enough, the westerly circulation will oscillate discontinuously between low and high exponential. There are three main reasons : (1) The bifurcation of baroclinic wave amplitude is the cause of the conversion of zonal circulation and meridional circulation in a relatively short period of time. (2) The nonlinear characteristics of the flow field in the middle and upper troposphere are the key to the dipole blocking situation, and the square wave characteristics of baroclinic wave amplitude change rate are the reasons for the meridional circulation with the dipole blocking situation that can maintain for a long time. (3) The amplitude and change rate of baroclinic waves control the development and extinction of the dipole blocking situation by periodically adjusting the magnitude of the nonlinear term, which is the reason for maintaining the infinite cycle of the entire westerly circulation between the zonal circulation and the meridional circulation.