Abstract: AerMet 100, an ultra-high-strength steel (A100 steel), is a promising material extensively utilized in the aerospace industry and military equipment for heavy-load components subjected to repeated impact loads frequently, leading to impact fatigue failure. This study, using a drop-weight impact testing machine, conducted the impact fatigue life tests on the three-point bending specimens with different notch types. A modified fatigue life prediction model for impact fracture energy loss was successfully applied for a unified fit. The crack initiation and propagation mechanisms under cyclic impact loads were investigated using SEM, EBSD, and non-contact DIC techniques. The investigation results revealed that: with the increasing impact energies, the impact fatigue life of A100 steel specimens with different notch types exhibited an approximately exponential decay. At the microscale, it is interpreted as the width of fatigue striations increasing almost exponentially with increasing impact energy. Regarding the damage mechanism, the impact failure mode of A100 steel was characterized by microvoid coalescence, with cracks nucleating at single or multiple points, and the predominantly crack propagation is along large austenite grain boundaries, accompanied by transgranular fracture through substantial martensite, leaving a trail of refined subgrains in the crack path. The first 50%~60% of the impact fatigue life (N_\mathrmf) is attributed to the crack nucleation stage, followed by 60%~90% N_\mathrmf to the stable crack propagation stage, with the final less than 10% N_\mathrmf marking the rapid crack propagation phase.