The compound channel is designed into different shadows of ladder-type according to different ship drafts. Small boats sail through the shallow side of the channel while big ships sail through the deep side. Because of the low ability in resisting wave, the small boat’s swing angle tends to increase significantly by the big ship’s wave. The excess breadth between ships is necessarily to be designed to reduce the impact of the ship wave.This paper analyzes the small boat’s swing angle which is affected by the ship wave under different parameters according to two methods (mathematical model and physical model) and judges whether the excess breath between ships can meet the requirements.