Scientific understanding of the hydrological and water conditions of reservoirs is of great significance for the rational formulation of flood control emergency plans and flood avoidance and transfer for reservoirs. Taking Yangtang Reservoir in Nanjing as an example, the hydrodynamic model is adopted to simulate the flood evolution characteristics in the case of dam failure. According to the calculation results of model, under the condition of dam failure, the total submerged area downstream of Yangtang Reservoir is 0.41 km². The submerged water depth is mainly less than 2 m, and its area accounts for 95%. The maximum submerged depth is 3.37 m, and the average submerged depth is 0.69 m. Combined with the population evacuation and transfer path formulated by the calculation results of the model, the flood disasters can be minimized and avoided, which will minimize the loss of people's lives and properties, and can provide the technical basis for the formulation of flood control emergency plans for reservoirs.