Abstract: Pointing at a large number of weld transverse crackings of 12Cr1MoVG water wall tube in the initial operation of 1000MW Ultra Supercritical Boiler. The causes of weld cracking of 12Cr1MoVG small diameter and thick wall pipe were analyzed by sampling macrography, chemical composition analysis, microstructure analysis, hardness test, scanning electron microscope observation, energy spectrum analysis and welding process investigation. The results show that the reason of weld transverse cracking is that some factors are not considered such as the environmental conditions crossing winter and spring in North China during the boiler installation time, the water wall tube’s diameter is small while the wall and thickness is thick, and the supply of material is minimum wall thickness（the minimum wall thickness is 5.8mm, and the actual wall thickness is more than 6.0mm）,etc.Due to preheating before welding and slow cooling after welding are not taken in the welding process, the heat input is too large, the interlayer temperature is not well controlled, and the cooling speed is too fast, the hardened martensite structure and reticular structure（high temperature pitting cracking） are found in the weld joint, the hardness of the weld joint joint exceeds or approaches the standard requirements, and the welding stress is large, all the factors lead to crack during welding. The welding quality has been effectively improved while the hidden danger of safety is eliminated by adopting some targeted measures such as electric heating preheating, interlayer temperature control, slow cooling and process supervision. At the same time, it reminds peers to pay attention to the welding process and the welding quality control of low alloy small diameter and thick wall pipe in the northern cold area.