Abstract: To explore the crack formation mechanism of solid self-piercing riveted joints of Mg/Al dissimilar alloys, the mechanical properties of the magnesium alloy materials were characterized. The material constitutive model and GISSMO fracture failure model of the magnesium alloy were constructed, and a refined simulation model for the forming process of solid self-piercing riveting was established. The strain field distribution and material flow characteristics during the joint forming process were analyzed; the internal crack formation mechanism of the joint was revealed, and crack suppression was achieved through die structure optimization. The simulation and experimental results indicate that cracks are mainly generated in the piercing and extrusion stages of the riveting process; crack sources are initiated in the lower magnesium alloy sheet near the inner and outer edges of the die boss and the dissimilar material interface; under the combined extrusion of the punch and die boss, the crack sources on the upper and lower surfaces of the magnesium alloy sheet propagate towards each other and finally form macroscopic “bowl-shaped” cracks. The optimized beveled die reduces the peak effective plastic strain in the dangerous area, slows down the material flow rate, suppresses the generation of joint cracks, improves the material filling performance in the mechanical interlock region, and realizes the efficient and reliable joining of Mg/Al dissimilar alloys.