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) process 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. Comparative experiments were conducted on ultra-high strength steel using both conventional and step-cooling laser welding processes. The mechanical properties of welded joints were systematically analyzed, and the strengthening-toughening mechanisms of step cooling laser welded joints were 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%, tensile strength by 4.9%, and toughness by approximately 6.8%. This improvement primarily stems from accelerated cooling rates during step cooling , which refine prior austenite grains and produce finer/narrower lath martensite structures, thereby enhancing joint strength. Concurrently, the self-partitioning effect increases retained austenite content, inducing transformation-induced plasticity (TRIP) effects that contribute to toughness enhancement.