Based on the measured data of the river evolution and the flood process in the Weihe River downstream after the building up of Sanmenxia Reservoir, the impacts of the Weihe River downstream channel sedimentation and shrinkage on flood characteristics were analyzed by combining the measured data analysis, the theoretical analysis and the numerical simulation. The results showed that the Weihe River downstream channel sedimentation and shrinkage caused significant changes in the evolution characteristics of the Weihe River flood, resulting in rising the flood level, prolonging the flood peak propagation time and increasing flood peak clipping rate of the Weihe downstream. And those are also the main reason for the Weihe River downstream sedimentation and shrinkage.Due to the change of water-sediment conditions in the Weihe River downstream after 2003, that is, the flood peak discharge significantly decreased and the river channel was flushed, the floodplain discharge decreased compared to the previous. It led to decline in the flood level, shortening in the flood peak propagation time and decrease in peak-clipping rate.