The existence of many medium and small reservoirs in the river basin changes the original process of runoff generation and confluence, which has more obvious impact on the arid and semi-arid regions of northern China. On the basis of DHF hydrological model which applied to the northern arid and semi-arid regions, this paper quantitatively estimates the capacity of runoff interception of medium and small reservoir swarm according to the data such as antecedent precipitation, evaporation, control area of small reservoirs, beneficial reservoir capacity, and some others; Considering the varied influences of medium and small reservoirs swarm on the flood confluence process of different soil moistures, distribution of rainfall and different cumulative rainfall,and then it built the improved DHF model and applied it to Menlou basin of Yantai .The simulation results show that the improved DHF hydrological model can effectively improve the accuracy and qualified rate of Menlou reservoir flood forecast and can provide scientific basis and technical support for reservoir flood control operation.