The stability analysis of high earth-rockfill dam is mostly limited to two-dimensional situation, and there is no suitable three-dimensional stability analysis method. In addition, the rockfill material of earth-rockfill dam is prone to strain softening under strong earthquakes, which affects the seismic performance of the dam. It is necessary to develop a three-dimensional stability analysis method for high dams that can take the softening effect into account. Firstly, a three-dimensional static and dynamic stability analysis method for high earth-rock dams is proposed based on the finite element sliding surface stress method combined with the optimized horn-shaped three-dimensional damage mechanism. Then, the relationship between post-peak strength and post-peak shear strain is obtained by processing the triaxial test results of rockfill in order to consider the impact of the softening effect of the rockfill material. Finally, the developed stability analysis framework is applied to analysis the static and dynamic stability of three-dimensional high earth-rockfill dam, and is compared with the two-dimensional case. The results show that the slippage with and without considering the softening effect differ significantly, and this difference increases as the cumulative slippage increases. Compared with the three-dimensional stability analysis, the two-dimensional stability analysis based on cumulative slippage tends to underestimate the seismic stability of the dam slope.