Abstract: The dynamic response analysis of high concrete dams under earthquake actions has always been a hot spot in the field of engineering seismic resistance. The analysis process exists many randomness. This work starts from the point of view of improving mesh accuracy and of increasing computation efficiency, a novel method for uncertainty quantification (UQ) in the seismic analysis of high concrete dams based on stochastic scaled boundary finite element method (SSBFEM) is proposed. In this method, octree mesh technique is used to discretize a high concrete dam-foundation-reservoir system. The SSBFEM is constructed to analyze the influence of random parameters on the dynamic response of the system. Additionally, the method proposed can integrate octree mesh and SSBFEM on commercial software ABAQUS user defined element (UEL), and it provides high-precision samples for UQ based on Monte Carlo simulation (MCs). For the time-consuming nature of MCs, a fast algorithm based on model order reduction (MOR) combined radial basis function (RBF) is proposed. Through the linear combination of the constructed low-order subspaces, the solution required for MCs is quickly obtained, and the computational efficiency of traditional MCs is improved. Finally, the accuracy and effectiveness of the UQ method proposed are verified by three different engineering examples.