Abstract: Reservoir bank collapse is a geological environmental disaster phenomenon with time-space boundary attributes. Time boundary refers to the time span, including long-term and short-term bank collapse. Space boundary refers to different reservoir sections. Differences in bank collapse patterns and prediction parameters are caused by differences in geological environmental conditions. At present, the short-term prediction time span of reservoir bank collapse commonly used is 2 to 10 years, but for engineering applications, there is a lack of sufficient decision-making basis, which brings difficulties to practical applications. Based on the characteristics of the reservoir water storage process, this paper proves that the selection of the short-term prediction time for bank collapse can be defined according to the time period of the reservoir water storage process. From the perspective of large-scale reservoir projects such as the Three Gorges and Xiaolangdi, the definition method is scientific and reasonable. And a decision-making flow chart for the time-space boundary of bank collapse prediction is designed. The decision-making theory of time-space boundary is applied to the short-term bank collapse prediction of Malianhe Reservoir in Gansu, and the decision-making process of its time-space boundary is expounded in detail. And from the perspective of engineering geology, suggestions are provided for the application of short-term bank collapse prediction results.