Abstract: Tensile tests were applied to five kinds of specimens with different shapes and notch poisitions in the MAG welded joint of 980 MPa low-carbon bainite high strength steel used for deep-sea. The dynamic fracture process of every kind of specimens was in-situ observed to analyze the fracture mechanism, and to determine the week part of joint.The results show that the fracture of straight notch specimens suffered the process of plastic deformation, initiating, propogation, blunting and fracture of cracks. The crack tip repeatedly blunted, prapogated, new cracks initiated, re-blunted, re-propagated during crack propogation. The arc and flat specimens fractured in a shear mode and suffered the process of plastic deformation, necking, initiation, propagation and fracture of microracks. The weak part of all specimens was the weld metal.