To explore the physical meaning of model parameters in Xin’anjiang-Haihe model and improve the simulation accuracy, by taking a part of Haihe River Basin above the Huangbizhuang reservoir as an example, the Xin’anjiang-Haihe model was established. The physical meaning of model parameters was qualitatively analyzed based on factors such as land surface characteristics, groundwater extraction, and water management facilities to clarify their roles in the runoff generation and flow concentration processes. Formulas for calculating the basin leakage based on the infiltration equation and water balance method were proposed to determine related model parameters. Subsequently, the model with calibrated parameters was used to conduct the flood simulation. The analysis revealed that the surface storage capacity of reservoirs could be considered as the storage capacity for surface runoff, while the subsurface storage capacity, influenced by factors such as groundwater over-extraction, represented the average water deficit in the basin after a storm event. The leakage coefficient represented the ratio between the leakage and the basin runoff during the flow concentration process. The simulation results of the model showed that compared with the original Xin’anjiang model, the proposed method improved the qualification rate of flood runoff depth increased by 42.9% and the qualification rate of peak discharge increased by 28.6% in Weishui basin, and by 33.4% for flood runoff depth and 33.3% for peak discharge in Pingshan basin.