Abstract: The stress state in welding of small diameter LSAW pipes is analyzed and the key causes leading to burning through, air holes and undercutting occurred in welding of small diameter LSAW pipes are summed up.The control measures to prevent the above defects occurred in welding joint are provided through analysis of the effects of the "pout" on the weld quality of small diameter and thin walled LSAW pipes.The result shows that the gravity and heat input are the main reasons which cause the weld defects occurred in small diameter and thin walled LSAW pipes.The weld defects occurred in inner welding joint of small diameter and thin walled LSAW pipes can be reduced by controlling the heat input, the "pout" during formation and inner welding joint width as well as properly adjusting the bevel dimension.