The aluminum / magnesium dissimilar metal composite structure has great application value in the field of structural lightweighting. In this paper, a novel friction stir spot welding - brazing (FSSW-B) technology was used for lap spot welding of aluminum / magnesium dissimilar metals. At the same time, it was compared with friction stir spot brazing (FSSB) technology to study the effect of the existence of the stirring pin in the welding tool on the microstructure and Mechanical properties of the joint. The middle layer of the interface of FSSW-B joint is divided into two parts: the upper interface is mainly composed of MgZn2, and the lower interface is mainly composed of Mg7Zn3; the FSSB joint is mainly composed of MgZn2 phase. The fracture mode of the joint is mainly interface peeling fracture, and the eyebrow-shaped fracture mode appears due to the existence of the stirring pin. The existence of the stirring stir improves the tensile shear resistance and fatigue performance of the joint. The maximum tensile shear force of the FSSW-B joint is 7600 N, and the fatigue limit is 3366.6 N; The contribution rate of stirring to tensile shear performance is 53.5%, and the contribution rate to fatigue performance is 11.4%; The existence of stirring pin in FSSW-B increases the dispersion of joint fatigue properties.