Because of the influence of human activities, the underlying surface of the Haihe River Basin has changed, which leads to the change of the runoff?characteristics. In this paper, to master the situation of the watershed underlying surface changes impact on the floods, we used the improved Xin'anjiang-Haihe model to study the Houwan Reservoir watershed. A great amount of historical data show that the Houwan Reservoir watershed underlying surface changes happened around 1980, the effect is?better when the improved Xin’anjiang-Haihe model being applied in the flood simulations at different magnitudes. Because of changes of the underlying surface, comparing to the parameter before 1980, the runoff parameter of the improved Xin’anjiang-Haihe model after 1980?has a tendency to increase, and under the same rainfall condition, the flood volume after 1980 shows a decreasing trend. What’s more, the smaller the flood magnitude, the greater the flood volume decreases.