Using the cloud imagery and NCEP reanalysis data,the work studies the characteristic of the cloud band and wind flow for the weakening of the 7th typhoon in 2002.The results show that the weakening of the typhoon is influenced by the asymmetric structure of the cloud band,high-level jet and divergence flowing into the typhoon.Besides,the research on weakening and re-intensification of the 4th typhoon in 2004 and the 16th typhoon in 2002 show that the development of U-component at the low level in the south of typhoon environment flow is helpful for re-intensification of the typhoon,but the development of V-component at the high-level in the west of typhoon environment flow is destructive for typhoon structure.