By using the daily precipitation data at 24 surface synoptic stations in southern Gansu Province during 1971-2010, the upperair sounding data in Wudu and NCEP reanalysis data, the changes in the extreme precipitation during the main flood season in southern Gansu Province and their causes are diagnostically analyzed. The results showed that extreme precipitation days in southern Gansu Province during the main flood season have interannual and interdecadal differences. Its possible causes include differences in atmospheric potential instability, caused by differences in atmospheric temperature and humidity fields, differences in moisture conditions, and differences in atmospheric dynamics. In years of more heavy precipitation days in main flood season in southern Gansu, temperature and relative humidity are higher in the lower atmosphere in this area, and lower in the upper atmosphere, the instability index K is greater, meaning that the potential instability of the atmosphere is stronger. And the lower level convergence and upper level divergence are stronger, which is conducive to upward motion, leading to a stronger probability of extreme precipitation occurrence.