The aim of this research is to investigate the adverse impact of heavy rainfall caused by water wing of slit-type bucket at the end of spillway. Parameters which could reflect the hazard degree of water wing are analyzed via model test. These parameters include the distance from water-wing-affected area to bucket outlet, the area enveloped by rainfall intensity contours, the elliptic shape coefficient, and the elliptic skewness coefficient. An expression of hazard intensity of water wing is also established to calculate and examine the measured rainfall intensity in the model test. Results findings suggest that when the Froude number of incoming flow increases and the angle of bottom plate decreases, the damage degree of water wing to spillway decays but to the downstream areas intensifies; when the contraction ratio of the slit-type bucket decreases, the damage degree of water wing to both the spillway and the downstream area intensifies. The research findings can be used to quantitatively evaluate the hazard intensity of water wing in physical model test and provide theoretical support for the selection of bucket shape parameters.