Multi-pipe inverted siphon is a common structure in water diversion projects. Outlets are usually kept a bit far away from the inverted siphon to keep the flow stable. However, due to difficulties in construction and land acquisition, the inverted siphon and lateral outlet at Xiazhuang are put together, posing adverse impact on flow pattern at the inlet. Based on physical modelling, a method of replacing pressure-free open channel outlet by pressurized tube outlet is proposed to minimize the impact of lateral outlet releasing and improving the flow pattern. Test reveal water surface oscillation with a maximum amplitude of 1.4 m at three downstream lock chambers under normal conditions. Analysis shows that the oscillation is induced by the vertical dispersion of flow between bulkhead gate shaft and working gate shaft. Arranging separating beam or slab could curb the vertical dispersion of flow, hence alleviating surface oscillation by reducing the amplitude to 0.1 m. Separating beam is recommended in future optimization as it is less costly in both engineering quantity and economy. The research findings offer reference for the design, research and operation management of similar projects.