Abstract: Y-groove cracking test was conducted to study the welding crack resistance of NM400 steel. Assessment of welding crack resistance of steel NM400 consists of the surface crack rate,section crack rate and root crack rate,and the effects of preheating on the crack resistance of steel NM400 has been analyzed. Thermal elastic-plastic finite element method was applied to carry out a finite numerical simulation of welding process for samples used in Y-groove cracking test and transverse welding residual stress distribution and peak position were obtained. The results indicated that the root crack rate of the steel NM400 at room temperature reached a high value,and it could be decreased to zero when preheating at 150℃. Therefore,preheating before welding can improve the welding crack resistance of steel NM400. Otherwise,stress concentration occured at the root of the Y-groove. Local transverse residual stress which was larger than the yield strength of materials is mechanical factors causing cracks.