The phase Ⅱ cofferdam of the Yakou Navigation Complex project on Hanjiang River is an overflow cofferdam with silty fine sand as the main filling material，supplemented by geotextile and plain concrete slabs to resist water scouring.After the flood in 2021，the concrete slabs on the cofferdam overflow surface suffer from local failure such as local void and fracture of connecting steel bars between slabs.This paper uses the nonlinear finite element method to conduct numerical simulation of the stress and deformation of the overflow cofferdam and analyzes the causes of the local failure of the cofferdam to propose preventive measures.The results show that under the combined action of gravity and water load，the non-uniform settlement deformation of the cofferdam overflow dam slope surface is too large，resulting in varying degrees of void between the anti-scour concrete slabs and the cofferdam body filling material.The stress state of the connecting reinforcement between the slabs changes from tension to shear，and local shear failure occurs due to insufficient strength，which ultimately leads to a reduction of the integrity of the concrete slabs and insufficient anti-flow scour capacity.Reducing the non-uniform settlement is the key to preventing such failure.Therefore，it is necessary to strengthen the filling and rolling to improve the compactness of the material，thus enhancing the anti-settlement deformation capacity of the dam body.Geomembrane reverse filter is set under the concrete slabs to improve the capacity of anti-seepage deformation and anti-flow scour.