Abstract: The branch exit flow at tidal river is significantly affected by high and low tide，and the flow pattern is complex. It is necessary to study the scouring condition under the sluice gate that is located at the estuary outlet of a branch tidal river, so as to prevent severe river erosion and avoid the damage to the adjacent buildings when conducting diversion or drainage of large flow. In this paper, taking Jiebei water conservancy engineering on Xinmeng River as a case, a 1:50 physical model was built to carry out a series of tests. The test results show that it is easy to cause serious scouring to the channel bed of Xinkai River, a tributary of Xinmeng River, if it is dug into the silty sand layer that has low incipient velocity; as the current generates vortexes at the edge of soft and hard media and it continually scour the bottom of the pit, a series of measures should be taken to protect the river bed in whole width of the river; the building protection design can mainly aim at the severe scouring area on the mainstream pass according to the hydrodynamic characteristics of the tidal flow.