The phenomenon of femtosecond laser ablation of convex structures from the bottom to the top is interesting. In this study, AZ31B magnesium alloy was used as the substrate to analyze the impact of the laser pulse energy and scanning speed on the morphology of concave-convex microstructures. Subsequently, a unified two-dimensional numerical model incorporating solid, liquid, and gas phases was established, and combined with experimental data, the mechanism and formation process of concave-convex transformation in magnesium alloy under laser ablation were revealed. The results indicate that the transition from concave to convex structures is significantly influenced by the laser scanning speed, whereas the laser pulse energy primarily affects the shape and size of the convex structures. During the ablation process, molten material is expelled and gradually accumulates on both sides of the ablation groove under the action of the recoil pressure. During cooling, the molten material at both ends of the groove merges to form protrusions under the combined effects of internal negative pressure, gravity, and Marangoni forces. Moreover, this method of femtosecond laser ablation for generating convex structures deviates from the traditional single-texture approach to concave structures, potentially broadening the application of laser composite processing surfaces.