Rare research has been reported on the water-wing in slit-type bucket caused by contracted sidewall which poses a threat on bank slope stability. In this article, a physical model is setup to investigate into the relation between the type of bucket (contraction ratio β and bucket angle θ) and the characteristic of water-wing for revealing the motion law of water-wing in slit-type bucket and the distribution characteristic of rainfall intensity formed by the water-wing near downstream. Results show that along with the decreasing of contraction ratio, the water-wing is ejected higher, and at the same time, the scope of rainfall is larger and its location is closer to the outlet; whereas along with the decreasing of bucket angle, the water-wing trajectory doesn’t change obviously, but the lateral diffusion aggravates, resulting in the increase of rainfall range. On this basis, a concept of secondary water-wing lateral diffusion coefficient is proposed to predict the shape and scope of rainfall caused by secondary water-wing in different slit-type buckets. The research results could enrich the research contents of slit-type bucket, expand research range, and provide more theory foundation for design.