Abstract: In the dam-reservoir dynamic fluid-structure coupling analysis, the scaled boundary finite element method (SBFEM) can simulate the semi-infinite reservoir water by discretizing only the interface of the fluid and the structure to save the number of node degrees of freedom and have higher efficiency. However, because the hydrodynamic pressure additional mass matrix is full, a large-scale elasto-plastic dynamic analysis of CFRD takes too much time for solving the equation. According to the physical meaning and distribution characteristics of the additional mass matrix of the hydrodynamic pressure by SBFEM, a simplified dam-reservoir dynamic coupling analysis method based on FEM-SBFEM is proposed. Only a retention coefficient β (0 ≤ β ≤ 1.0) is needed to simplify the hydrodynamic pressure additional mass matrix in different degrees, which is simple and easy. The simplified method is applied to the calculation of the dynamic elasto-plastic coupling of CFRD and reservoir water. The value range of β is suggested to improve the computational efficiency with good accuracy.