Abstract: This paper deals with the initiation and propagation of hydrogen induced crack while 10CrNi3MoV steel of 590 MPa grade was welded by MIG welding using WM960 S wire and Ar+20%CO₂ mixed gas without preheating.Tekken cracking test was operated under the condition of -5℃ and 80% relative humidity.Compared between the single and the multipass welding,the mode and rate of cracking propagation demonstrated that weld metal has good capability to resist crack propagation.Multipass welding with transcrystalline crack possesses better crack resistance than single pass welding with intercrystalline crack.Because of the effect of heat cycle by the second pass in multipass welding fine grains occurred at both sides of coarsed grain boundary in the root of joint,which can change the mode and rate of crack propagation.According to the mode of crack propagation the mechanism of crack initiation and propagation was also discussed.