On-site investigation and numerical simulation are used to find out the reasons of ground collapse in the rear of a seawall.Firstly,the reasons of ground collapse are initially discussed according to results of on-site investigation.Secondly,constitutive model of seepage in saturated soil is built up，by which the seepage gradient of the interface between land backfilling materials and seawall material is solved by finite element method.Seepage failure is judged by making a comparison of the simulated seepage gradient and allowable hydraulic gradient,therefore,the reasons of ground collapse can be further verified.Finally,effective measures are put forward by the reason analysis.Results show ground collapse is caused by seepage failure of backfilling material.The ground collapse is caused by the infiltration failure of backfill,which is the result of the combined action of the inverted filter structure damage on the inner slope of the seawall and the precipitation infiltration in heavy rainfall weather.The further development of the ground collapse can be effectively prevented by mending the filter structure and preventing rainfall infiltration.The research provides an important reference for anti-seepage design and ground collapse mending.