Solid-state welding mechanism of ultra-high strength C-Mn-Si martensitic steel during friction stir spot welding (FSSW) was clarified and interface strengthening principle was revealed. We found that the generated oxides in the interface became spherical and dispersed, which was caused by selective oxidation due to the reduced oxygen partial pressure during high welding temperature. Then, these oxides were further refined and dispersed by severe material flow around the welding interface. Consequently, the refined and spherical (Mn, Si, Al)O amorphous oxides were formed. In addition, strong material flow introduced large drive force for grain boundary migration around the welding interface, which further facilitated the migration and dispersion of the generated (Mn, Si, Al)O oxides, giving rise to a high-strength welding interface.