To solve the coupled vibration of a gravity dam-reservoir system with variable waterdepth by using a hybrid element method,the fluid region with variable water depth needs to be dis-cretized by FE meshes.However,such a method asks for a great computational cost owing to the ex-cessive unknowns,especially when the fluid region with variable water depth is relatively large.Toovercome the shortcoming,a refined boundary element method is proposed to analyze the fluid field,in which only the discretization for the boundary of the variable depth region is required.But as a basisof this approach,it is necessary to construct a new Green’s function corresponding to an infinite stripregion.The problem is solved as the first step in this paper by employing Fridman’s operator functiontheory,and then a mixed FE-BE formulation for analyzing the free vibration of the gravity dam-reservoir system is derived by means of the coupling conditions on the dam-reservoir interface.Final-ly,a numerical example is provided to illustrate a great improvement of the method developed hereinover the hybrid element method. To solve the coupled vibration of a gravity dam-reservoir system with variable water depth by using a hybrid element method, the fluid region with variable water depth needs to be dis-cretized by FE meshes. However such a method asks for a great computational cost due to the ex-cessive unknowns, especially when the fluid region with variable water depth is relatively large. Toover the shortcoming, a refined boundary element method is proposed to analyze the fluid field, in which only the discretization for the boundary of the variable depth region is required.But as a basis of this approach, it is necessary to construct a new Green’s function corresponding to an infinite stripregion. The problem is solved as the first step in this paper by employing Fridman’s operator function theory, and then a mixed FE-BE formulation for analyzing the free vibration of the gravity dam-reservoir system is derived by means of the coupling conditions on the dam-reservoir interface. Final-ly, a numerical example is provided to illustrate a great improvement of the method developed hereinover the hybrid element method.