Based on NCEP/NCAR reanalysis data (0.25°×0.25°), FY-2G Temperature of Brightness Blackbody (TBB) product, dual-polarimetric radar data, the hourly observations from Zhejiang Automatic Weather Stations (AWS), the causes of extraordinary rainstorm by typhoon "Lekima" (1909) in Zhejiang Province are diagnosed. The results are as follows:(1) The extreme precipitation in Zhejiang Province was induced by deep body circulation of "Lekima", which has the characteristics of wide coverage range, large total amount and extreme local rainfall intensity. Topography has a significant effect on local heavy rainfall reinforcement. (2) The moisture flux at 850 hPa, the convergence at 850 hPa and the divergence at 200 hPa all exceed the climate average by 3-4 standard deviations either before or after the typhoon landing, moreover, the abnormally strong dynamic uplift and moisture transport provide favorable conditions for extreme rainfall. The anomaly of physical quantities plays an important factor in judging extreme precipitation. (3) The active southwest monsoon and the easterly jet stream in the south of the subtropical high provide sufficient vapor and energy for typhoon"Lekima". The shape and location of the large-value region of water vapor flux at 925 hPa corresponds well with those of the rainstorm area, and the variation of convergence intensity has certain indication significance for precipitation. (4) The enclosed cloud area in the typhoon center before and after landing is large in scope and compact in structure, with several mesoscale convective systems developing strongly and moving slowly, which is the main reason for extreme precipitation in the eastern coastal areas of Zhejiang. The precipitation particle phase identification products of dual-polarization radar have high reference value in nowcasting forecast. (5) The weak cold air in the middle troposphere and warm wet air in the lower layer promote the development and maintenance of instability convective stratification. With the forced uplift of ground mesoscale convergence lines and topography, mesoscale convective system is constantly triggered and the train-effect is occurred,which is also an important factor for the local extraordinary rainstorm in northwest mountainous areas in this process.