With the improvement of units capacity and parameters of the coal fired power plant, the alloy steel materials are wildly applied in water-cooled wall tubes of the ultra supercritical boiler u- nits, so several welding crack failures happened during the alloy steel water-cooled wall installation of the ultra supercritical boiler units in winter construction in North cold area, which seriously influ- enced the project quality and progress. By the comprehensive analysis of welding process , environmen- tal temperature , preheating before welding , metallographic organization of the welding joint and the form of the welding crack, the causes of the crack were found out, the measures of crack prevention were presented.