Abstract: As short upper trough is considered as a wave packet in baroclinic flow, the evolution of the wave packet is investigated as an initial value problem in a quasi-geostrophic model, it is found that evolution of the wave packet depends on the packet' s structure. When the wave packet under the jet stream inclines to the west at initial stage. it will develop quickly. While the wave packet under the jet stream inclines to the east. it will be decaying. These conclusions are consistent with those obtained from WKBJ method 1 However, whether the wave packet is west tiltor east tilt, it can excite unstable baroclinic long waves in the baroclinic flow besides the evolution of the wave packet itself. Furthermore, along with the evolution of disturbances the most unstable baroclinic wave becomes a dominator. These results may be used to explain the generation and development of some midiatitude cyclones.