Abstract: In this paper,the welding thermal cycle process of deep-sea pipeline steel was investigated using welding thermal simulation.The microstructure variation,crystallography characteristic and second-phase precipitation behaviors of the softening zone of heat affected zone（HAZ） were characterized and analyzed by combining with the scanning electron microscope,electron back-scattered diffraction,transmission electron microscopy and hardness testing.The results show that HAZ softening zone appears in the fine-grained zone with a peak temperature of 1 000～900 ℃ for deep-sea pipeline steel whose base metal microstructure is the polygonal ferrite and acicular ferrite.Using V microalloying and low welding heat input can reduce the grain size,increase the proportion of small-angle grain boundaries and the dislocation density effectively.Thus,the softening problem of the HAZ fine-grained region can be effectively improved.