Abstract: To address the strength-toughness mismatch in ultra-high strength steel welded joints, a step-cooling process inspired by the Quenching and Partitioning (Q&P) principle was proposed and its impact on the mechanical properties of welded joints was investigated. The step-cooling system was designed using welding numerical simulation methods and integrated with existing laser welding equipment. The conventional laser welding tests and the step-cooling laser welding tests were conducted on ultra-high strength steel. The influence of step-cooling laser welding process on the mechanical properties of welded joints was compared and analyzed. Additionally, the strengthening-toughening mechanism of step-cooling laser welded joints was elucidated. Results demonstrate that compared to conventional laser welding, the step-cooling laser welding reduces the width of various joint zones while enhancing yield strength by 5.8%, increasing tensile strength by 4.9%, and improving toughness by approximately 6.8%. This improvement primarily stems from accelerated cooling rates during the step-cooling process, which refine prior austenite grains and produce finer/narrower lath martensite, thereby enhancing joint strength. Moreover, the self-partitioning effect increases retained austenite content, inducing transformation-induced plasticity (TRIP) effect that contributes to toughness enhancement.