Abstract: Based on conventional weather data and the new generation Doppler Radar data, the causes of an extreme rainstorm in Kaijiang, Dazhou from June 17th to 18th, 2021 are analyzed. The results showed this rainstorm was under the joint influence of subtropical high-pressure belt's eastward retreat and southward shift, low trough's eastward shift, low level shear, low level jet stream's eastward and southward shifts, as well as ground cold air; train effect formed by warm cloud precipitation echo of low mass center was the direct cause of extreme precipitation; when heavy rainfall weather was predicted, it's necessary to understand the relationship between the actual rainfall situation in the upstream and the influcence system, as well as the performance of the model, so as to appropriately rectified the rainfall area and magnitude of heavy rainfall in the downstream by combining with own experience; the model itself had errors, and extreme precipitation was difficult to predict, but it's possible to effectively rectify the heavy rainfall area according to the latest actual data, and provice short-time impending prediction service in time to make up for the deficiency of short-term prediction.