In response to the “23·7” extreme storm-flood event in the Haihe River basin, the Beixindian watershed of the Qingshui River was selected as the study area. Based on the grid Xin’anjiang model, a dual retention grid model was developed by coupling surface and subsurface retention modules. The “23·7” flood process was retrospectively simulated, and the rainfall-soil water-groundwater response relationship and the regulation and storage effects of the surface and subsurface retention reservoirs were analyzed. The results show that for 11 historical flood events from 1980 to 2000, the dual retention grid model achieves a 100% qualification rate for runoff depth and a 64% qualification rate for flood peaks, and the proportion of events with a deterministic coefficient greater than 0.5 is 64%, which are superior to the grid Xin’anjiang model and the existing forecasting schemes in the study basin. For the “23·7” flood process, the simulated errors of flood peak and runoff depth by the dual retention grid model are constrained within 4%, with a deterministic coefficient of 0.91. The spatiotemporal variation of soil moisture has a regulating effect on the formation and recession processes of the “23·7” flood. The surface retention module of the dual retention grid model has a flood-retention and peak-reduction effect, and the subsurface retention module improves the agreement of the recession curve.