Abstract: Dry intrusion plays an important role in the explosive development of cyclones and the evolution of cold fronts.Characteristics of dry intrusion during a rainfall event that occurred in Northern China is analyzed in detail. It is found that dry intrusion maintained during the precipitation periods. The intensity of precipitation increased with the increasing dry intrusion, which pushed the rainy region southeastwards. Based on a previous study by McNulty, the IM (ingredientsbased methodology) developed by Doswell, Schultz and Schumacher is utilized in this study. In this paper, all the physical representations such as low relative humidity, and cold advection, and high potential vorticity (on either isobaric or isentropic surfaces ) defined in the past studies of dry intrusion are combined into a simple and convenient parameter to characterize dry intrusion. This is a new attempt to extend the IM method in the research of heavy rainfall related to dry intrusion. The new dry intrusion parameter is used to analyze the isentropic evolution of dry intrusion during a rainfall event. The analysis shows that, this parameter can better quantify the intensity of dry intrusion and diagnose its evolution through using satellite infrared and water vapor imageries. By the contrast results on the isentropic surface and the corresponding isobaric surface, it is inferred that the analyses of dry intrusion on both surfaces are consistent with each other. By the isentropic analysis of dry intrusion, cold and dry air at upper levels overruns that in the lower troposphere, where moist and warm air is located. Thus, potential instability is built up in the vertical direction, which is in favor of the occurrence of precipitation. Therefore, we may look for dry intrusion regions by tracing strong signals of the dry intrusion parameter, and focus on the instability near dry intrusion regions in precipitation forecasting. This method will aid to improve the accuracy of precipitation forecasting.