Abstract: The "23·7" basin-wide rainstorm and flood process revealed that there is a certain correlation between the incoming water of the Zhaitang Reservoir on the Qingshui River, a tributary of the Yongding River, and the previous rainfall in the basin. In order to clarify the law between the runoff yield and concentration of the reservoir and the previous rainfall in the basin, a hydrological model of the Zhaitang Reservoir basin was constructed based on the HEC-HMS software, and the runoff yield and concentration process of historical rainfall and flood events was simulated and analyzed. The results show that the previous rainfall has a decisive impact on the timing of runoff yield. 45mm is the average rainfall threshold for runoff yield in the basin, and it fluctuates depending on the previous soil moisture content; the drier the previous soil is, the larger the threshold is. The determination of the critical rainfall threshold for runoff yield of the reservoir has important reference significance for understanding the runoff yield and concentration law of the basin, the rapid judgment of the incoming water of the reservoir, the flood dispatch and the timing of related defense work arrangements.