Abstract: The precipitation and characteristics of inclusions in DH36 steel after titanium treatment were determined by FactSage 7.1 thermodynamic software calculation and scanning electron microscope observation, and the welding thermal cycle process of 220, 253, 304 and 352 kJ/cm were simulated by thermecmaster-100KN thermal simulation test machine and the change rules of microstructure and low temperature impact toughness of HAZ were studied.The results show that the main inclusion types after titanium treatment are TiN, Al₂O₃, MnS and Ti-Al-O-Mn-S composite inclusions, in which the TiN particles less than 1 μm account for 67% of the total TiN particles.The microstructure of HAZ is polygonal ferrite, pearlite and granular bainite; the grain size at 253 kJ/cm heat input is the smallest, and the impact toughness at -20℃is the highest, and then with the further increase of heat input, the grains coarsen and the impact toughness decreases.