Torrential rain on August the fifth 2001 in Shanghai is diagnosed.Results show that thermal and water vapor background in Shanghai have changed 24 hours before,a water vapor transportation center has also formed.The dynamic condition imposed 12 hour ago,a large positive vorticity center at the middle and low levels also moved above Shanghai and its nearby region,this made instability energy release and formed the torrential rain.Furthermore,large positive vorticity at the middle and low levels and large negative vorticity at the high level overlapped in such a narrow region is also a inducement of the torrential rain.