Abstract: By using ECMWF day–to–day grid data, the distribution of vector E, the special property of kinetic energy and the features of the barotropic instability for 30–50 day oscillation are analyzed. It is found that the energy propagation of 30–50 day disturbance is closely related to the location of the west wind jet. The wave energy propagates zonally at the place where the west wind is stronger but meridionally at the place where that is weaker. Due to the stronger conversion of barotropic energy, the disturbance with low—frequency can get energy from basic flow, which causes the greatest kinetic energy of these disturbances in the jet exit region. The vertical distributions of kinetic energy here show us the fairly barotropic characteristics which strongly contrasted to the baroclinic over area of low latitudes