11 physical parameters including water vapor,energy,dynamics and unstable conditions were studied by using sounding data and conventional observation data for 196 heavy rainfall cases in Southwest China from 2010 to 2020. The thresholds of key environmental parameters conducive to heavy rainfall cases are extracted with box and whisker plots.The conclusions are shown as follows: :(1) The nearly saturated atmosphere and high specific humidity in the middle and lower troposphere are one of the favorable conditions for the occurrence of rainstorm in this region; (2) The threshold value of SI is lower in all seasons.The value of CAPE is higher in mid-summer, and convective rainstorms are more common, while stable rainstorms are more common in early autumn；(3) The requirements for EHI in early summer and mid-summer are obviously higher than those in early autumn, while EHI is only required in early summer.