Abstract: For achieving joint with high performance, mechanism of cracks in resistance spot welded magnesium alloy joint was investigated by optical microscopy,scanning electron microscope and energy dispersive spectrum,respectively. The formation of solidification cracking is caused by low melting point liquid films between dendrites due to segregation of Al and Mn atoms and tensile stress developed during cooling. Spot welding parameters (heat input) have a significant effect on susceptibility of the weld nugget to solidification cracking. The sensibility of solidification cracking increases with heat input. Because the larger heat input,the larger tensile stress caused by the shrinkage of the nugget is formed,and the lower melting point of liquid films is due to solute atom segregation with greater concentration and prolonging time for tensile stress affecting. It is favorable to select relatively low heat input for reducing sensibility of welded joint to solidification cracking.