Abstract: In this paper, a diagnostic study of the spring cyclone developed rapidly was made. This kind of cyclone is very similar to "Bomb" in the North America. The weather caused by the cyclone was severe and the violent wind, tornadoes, hailstorms and heavy rain were observed inside it. The results show that the occurrence and development of this kind cyclone is related, to the interaction between the cumulus convection and synoptic scale system in large scale course of baroclinie instability. In addition, it is found that the small scale cumulus merge frequently to form mesoscale systems, and that the instability of CISK plays a important role in the releace of condensation latent heating. On the other hand, it was discovered that the subtropical jet is very strong and the coupling between low and upper level jets is quite different from the studied previous. Finally, it is should be emphasised that the rapidly northeastwards moveing of the cyclone was caused by the asymmetric structure of the cyclone and in the meantime the asymmetric structure of cyclone is in favour of collection of cloud droplets to form raindrops, therefore, it is helpful to occurrence of heavy rain.