Of the 17,160 agricultural reservoirs nationwide, 96% are aged more than 30 years after construction. Aging reservoirs are at risk of deterioration and potential collapse. The torrential rains between July and August 2020 caused 26 collapse and loss filure across the country. Preemptive and comprehensive response plan is needed to prevent damage to life and property caused by such a reservoir collapse, and this study analyzes the early warning system of agricultural reservoirs as part of a preemptive and comprehensive countermeasure plan to diagnose the current situation and come up with improvement measures. Using bathymetric data, real-time water level change through water level-rainfall linkage analysis and early warning standard setting according to rainfall were proposed. Using bathymetric and aerial survey data, a three-dimensional reservoir collapse simulation was used to predict the arrival time and submerged range of the collapsed flow, suggesting early warning standards and positioning methods.