By using the conventional meteorological data,NCEP 1°×1° reanalysis data,Satellite Images data and CINRAD/SA data,the reason for the failure of the foecast and the cause of the heavy rainfall after Saola Typhoon landing is analyzed.Asymmetric distribution of precipitation by Typhoon Saola has been analyzed from circulation background,change of vertical structure,steering flows,environmental vertical windshear,physical quantity diagnosis and so on.The results show that:The main reason for the large deviation of typhoon rainstorm forecast and actual situation is that little attention has been paid to the change of the vertical structure of typhoon and its impact on the precipitation area during the forecasting process.The stable circulation background of high latitude provides favorable conditions of asymmetric distribution of typhoon precipitation;the change of steering flows in the vertical tilted the typhoon center southward with height;the steering flows and environmental vertical wind shear are the reasons which caused the typhoon center southward with height;The change of the vertical wind shear indicates the heavy rainfall region,which reflects the stronger convection and rain on the left of downshear;In addition,while the typhoon center southward with height,the distribution of the vorticity has to been changed,so the heavy rainfall happened in the southern Fujian proivince.Therefore,it is important to pay attention to the change of this vertical structure of typhoon.