A method is developed in this study to monitor and detect extreme precipitation events. For a rainfall event to be severe, it should last for a long period and affect a wide region while maintaining a strong intensity. However, if the duration is inappropriately taken as too long and the region is inappropriately taken as too wide, then the averaged intensity might be too weak. There should be a balance among the three quantities. Based upon understanding of the issue, the authors proposed a simple mathematical model, which contains two reasonable constraints. The relation of the “extreme” intensity with both duration and region (EIDR) is derived. With the prescribed baseline extreme intensities, the authors calculate the relative intensities with the data. Through comparison among different time periods and spatial sizes, one can identify the event that is most extreme, with its starting time, duration, and geographic region being determined. Procedures for monitoring the extreme event are provided. As an example, the extreme event contained in the 1991 persistent heavy rainfall over east China is detected.