Taking a hydraulic project in southwest China as an example, the influence of the dam head design on the earthquake failure process and ultimate seismic capacity of high gravity dams is investigated. In this paper, the Westergaard method is adopted to simulate the influence of the reservoir, and the Concrete Damaged Plasticity model is used to simulate the mechanical behaviour of concrete materials. Based on the overloading method, the nonlinear seismic response of high gravity dams under strong earthquakes is calculated. The influence of the dam head design on the seismic performance of high gravity dams is comprehensively analyzed by failure areas of the dam body, the relative displacement at the dam crest, and the sliding stability of the dam. The results show that the failure areas of the wide-head section and the narrow-head section under earthquakes are basically the same, located at the dam heel, the upstream face slope change, and the downstream face. The failure area of the wide-head section also emerges at the downstream slope change. Under the same ground acceleration, the failure area of the wide-head section is larger. Based on the sliding stability and the penetration of failure areas, it can be concluded that compared with the narrow-head section, the ultimate seismic capacity of the wide-head section is usually reduced. The reduction values of under three ground accelerations are 0.06 g, 0.09 g and 0.05 g, respectively.