This paper is to study the stress corrosion crack propagation behavior and mechanism of 12Cr1MoV welded joints in NaOH solution. The stress corrosion crack propagation behavior of 12Cr1MoV welded joints under different initial stress intensity factors (40, 50 and 60 MPa.mm1/2) in NaOH solution (80 ℃, 20wt.%) was carried out by three-point bending loading method. The crack propagation path and fracture morphology were analyzed by SEM and electron back-scatter diffraction (EBSD). The growth rate of stress corrosion crack increased with the increase of the initial stress intensity factor, and the maximum reaches 11.189′10–7 mm/s, but the growth trend slowed down gradually. The stress corrosion crack propagated in a dendritic manner, and the larger the stress intensity factor was, the earlier the main crack branched and the wider the crack expanded. Partial discontinuous cracks and “jumping” micro-cracks also generated during the propagation of the main crack. At high stress intensity factor condition, the fracture surface of the crack presented ice-sugar morphology, which belonged to the intergranular fracture. Under the condition of lower stress intensity factor (KI = 40 MPa.mm1/2), the fracture presented most intercrystalline assistant with some transgranular. With the increase of initial stress intensity factor is, the growth rate of stress corrosion crack increases, and the earlier the main crack branches, the wider the crack grows. Under the joint action of anodic dissolution and hydrogen-induced cracking, the stress corrosion crack of the weld propagates along the original austenite grain boundary, of which the anodic dissolution is the main one.