A new B-Al-rich steel with high-strength and plasticity was developed and its RSW performance was studied when used as the structure materials of automobiles. The hybrid orthogonal experiments of complex welding conditions were implemented under the ISO14327-2004 standard and the main factors that can influence the spattering of nuggets were found. The boundary curves of nugget-spatter lobe diagram that could effectively predicate the expulsion rate of the developed steel were also drawn. The relationships among the nugget size, spatter, welding process and environments were further understood. A moment balance function of nugget was developed and the function-based numerical simulation for the coefficient of swelling stress was conducted. Therefore, the further understood spatter mechanism promoted the engineering application of the developed hot-stamped steels.