The main reasons for incomplete penetration are small groove angles, too small gaps or too thick blunt edges, too small flame energy rate or too fast welding speed, too fast heat dissipation rate of the weldment, and the existence of the molten pool for a short time. Can not be fully fused with the base metal. The prevention method is to choose the correct groove form and proper welding assembly gap, and eliminate the dirt and slag on both sides of the groove and between the welding layers. Choose the appropriate flame energy rate and welding speed, which is good for fast heat conduction and large heat dissipation surface. Weldments require preheating before welding or heating during welding.