Abstract: A model for the water fracture modeling at dam-foundation interface is proposed based on Scaled Boundary Polygons and Cohesive Crack Model. Both the dam and foundation rock are modelled by Scaled Boundary polygons, while the fracture process zone of the interfacial crack is modeled by interface elements embodying a cohesive softening law. Since interfacial crack is always in mixed mode loading, interface elements are introduced in both the normal and tangential directions. The stress intensity factors are solved for cracks subscribed with arbitrary side-face tractions. The displacement field and stress field are solved semi-analytically, contributing to the high precision of the solution without asymptotic enrichment or local mesh refinement. The polygon generation and remeshing technique facilitates convenient modeling of the crack propagation process. The proposed model is verified by a numerical example, which also shows that the water pressure and its distribution pattern inside the crack influence remarkably the stability of the crack.