Abstract: In order to further study the drop zone of torrential rain in Weifang and summarize the experience of forecasting torrential rain in the future, the author made especially a diagnosis about the process of the torrential rain. The author took the method and had an analysis of profile and sounding to the conventional meteorological data by using a meteorological information comprehensive analysis and processing system 3.1 and had finally an inspection of numerical weather prediction, which got the reason of production and omission of torrential rain. The results showed that: the important reason of the torrential rain with the southern Yishan and indemnity river was commonly effected by cold air brought by the middle latitude shortwave trough and the southwest warm wet airflow brought by the jet stream. It was the main reason of the omission of torrential rain that the author relied excessively on the rainfall forecast of numerical products and neglected the evolution of small scale process as well as analysis of the physical quantity field. Above all, the author could draw the conclusion that it was very important to monitor the actual weather situation and analyze all kinds of physical quantities for forecasting torrential rain. Besides, people should improve forecaster to revise the work of the numerical weather prediction.