Large-scale background, environment field and triggering, maintaining mechanism are diagnosed with the observation data and NCEP 6-hourly reanalysis data with resolution 1°×1°. The results show that erectness of coupling pattern between upper-layer jet and low jet and a divergence area, which came forth in right of upper-layer jet provide favorable surrounding conditions. It's vital for triggering the rainfall that warm and moist southerly airflow erupt from low layer to the upper-mid of troposphere. Latent heat release from vapor condensation plays positive feedback to the ascending air. It's a key for the heavy rainfall maintaining and development that a strong pump at 200hPa exists, which interacts with vapor condensation heating.