Abstract: For dam transient analysis subjected to horizontal ground excitations and based on the scaled boundary finite element method (SBFEM), a dynamic stiffness of semi-infinite reservoir with constant cross section was proposed, which was evaluated by the Bessel function. Based on the dynamic stiffness, a SBFEM and finite element method (FEM) coupling equation was developed to analyze transient responses of semi-infinite reservoir with arbitrary geometry under horizontal ground excitations, where the semi-infinite reservoir was divided into two parts: near field and far field (semi-infinite reservoir with constant cross section). The near field with arbitrary geometry was discretized by FEM, while the far field was modeled by the SBFEM. Through comparing the calculation efficiency of the dynamic stiffness matrix and that of the dynamic mass matrix, it was found that the calculation efficiency of the dynamic stiffness matrix was higher than that of the dynamic mass matrix, and both of them had the same accuracy. Numerical examples showed the accuracy of the developed dynamic stiffness matrix and its coupled equation.