Since more and more high and steep rockfill slopes appear in engineering, the stability, failure process and influence range are critical to the engineering operation safety. The discrete element method was used to simulate the failure process of a high and steep rockfill slope. Taking a pumped storage power station project as an example, a discrete element model was established, and the failure process under the limit condition was simulated. The movement, retaining wall stress and change of the slope were analysed in detail. According to the variation of the average unbalanced force ratio, the failure process of the rockfill slope can be divided into five stages. The change of slope shape, displacement and velocity in each phase were mainly analysed. Besides, the instability mechanism of the rockfill slope was further clarified by monitoring the energy change and the stress change of the concrete retaining wall. Comparing with the displacement of accumulation body and the number of rock blocks crossing the specific position, the protection effect of the retaining wall was demonstrated. Furthermore, through quantitative analysis of the number of rock blocks reaching the riverbed and the force changes on the retaining wall, the concrete retaining wall at the bottom of the rockfill slope should be strengthened urgently. Through the discrete element numerical simulation, the failure process and influence range of the high and steep rockfill slope are defined and the suggested height of the concrete retaining wall is proposed.