The formation mechanism,influencing factors and control principles of cold lap defects in tandem GMAW welds were discussed. The results show that three types of cold lap are found at the weld toe of the tandem GMAW,their properties belong to the micro lack of fusion defects at the weld toe and may affect the fatigue life of the structure. Welding spatter and the overflow of molten metal are necessary conditions for forming cold lap,and the temperature of spatter,base metal and spilled metal are the sufficient condition for forming cold lap. The relationships between the welding behavior and the cold laps formation in tandem GMAW are established,the key points are that when the stability of arc and the balance of weld pool force are destroyed,the tendency of cold lap defect formation is increased. Four possible process routes to limit the number and size of defects are proposed.