In the process of actual operation of reservoir flood control, the operation risk is directly affected by the natural feature of flood. In order to reduce the risk,the paper proposed the forecast and operation model of combined flood control based on composition method of the same frequency region.Taking the minimum of sum of the maximum flood control capacity as target,the paper got the risk indicator of operation consist of combination of m kinds of floods and n types of design frequency of combined operation and conventional operation through flood calculation.Result showed that the combined flood control operation reduce the water level in front of the dam and the maximum discharge compared to conventional scheduling. Therefore, the new method reduced risk operation. In light of the operation model of combined flood control,the paper got combined discharge operation mode in the whole scheduling period by applying dynamic programming method.