There has to date been no quantitative analysis of the anti-sliding effects of protective measures implemented in high steep rockfill slope engineering under conditions of instability, despite the importance of this analysis for engineering design, improvement to engineering measures, and promotion of hazard warning and management. This study established a discrete element model (DEM) of a high steep rockfill slope based on an engineering project. Numerical simulation was used to investigate the anti-sliding effect of concrete retaining wall height on the high rockfill slope under conditions of instability, with a focus on the distributions of rock before and after slope instability, rock displacement, the quantity of rock crossing the concrete retaining wall, and the quantity of rock reaching the riverbed. The trajectory of rolling rock changes in velocity under different concrete retaining wall heights, and the resistance to rolling rock by the retaining wall were analyzed. Movement trajectories and velocity time-history curves of rolling rock suggested that a higher concrete retaining wall could enhance slope stability under long-term operation. Quantitative analyses determined the optimum height of the concrete retaining wall. The method proposed in this study could provide guidance for effective protective measures in high steep rockfill slope engineering.