Abstract: The defects of new light material Mg alloy in resistance spot welding(RSW) were studied.Their appearance feature,formation causes and damages were analyzed,which offers the proof to improve the quality of joint in resistance spot welding of Mg alloy.The results shows that cracks in the joint of RSW of Mg alloy are solidification crack,which are dependent on the welding current character,stress condition and component segregation at solidification.Shrinkage porosity and shrinkage cavity have relation with heat expansion of metal in welding process and forge force in later welding time.Electrode stick and expulsion are the most common defects in RSW of Mg alloy,which are caused by the local overheat at the interface of electrode-workpiece and workpiece-workpiece.