Experimental investigations on the DC spot welding of Mg alloy AZ31B are presented. Experiments are carried out to study the influence of spot welding parameters (electrode force, welding heat input and welding time) on the tensile shear load and the diameter of nugget, based on an orthogonal test and analysis method. The optimum parameters are as follows: electrode force is 2000N, welding heat input is 80% and welding time is 6 cycles. The microstructure of spot weld is single fine equiaxed crystals in the nugget, of which the structure is β-Mg_ 17 Al_ 12 precipitated on α-Mg boundaries induced by non-equilibrium freezing. And the surface condition of the workpiece has great influence on the joint quality. Experiments are carried out to study the influence of spot welding parameters (electrode force, welding heat input and welding time) on the tensile shear load and the diameter of nugget, based on An orthogonal test and analysis method. The optimum parameters are as follows: electrode force is 2000N, welding heat input is 80% and welding time is 6 cycles. The microstructure of spot weld is single fine equiaxed crystals in the nugget, of which the structure is β-Mg 17 Al 12 precipitated on α-Mg boundaries induced by non-equilibrium freezing. And the surface condition of the workpiece has great influence on the joint quality.