Abstract: In view of the problems of poor low-temperature impact toughness and non-uniform weld microstructure in X80 pipeline steel, X80 pipeline steel was welded by pulsed MAG welding with the addition of nano TiO₂, and the effect of adding nano TiO₂ on the weld microstructure and mechanical properties was investigated by inducing the nucleation of acicular ferrite through the in-situ synthesis of oxide inclusions. The results indicate that the weld microstructure with added nano TiO₂ is mainly composed of acicular ferrite (AF), granular bainite (GB), and polygonal ferrite (PF). The Mn-Ti composite oxides induced by nano TiO₂ serve as effective nucleation sites, driving AF to nucleate radially and increasing its content. The grain size is refined from 8.97 um to 5.14 um; the proportion of grain boundary angles greater than or equal to 45° increases by 7.5%; the proportion of low KAM values of less than 1 increases. The nucleation of AF promotes an increase in local dislocation density, which leads to higher partial hardness values. The average tensile strength of the weld with added nano TiO₂ reaches 722.67 MPa, and the average low-temperature impact absorption energy at − 40 °C is 82.91 J, which is an increase of 22.32%; the fracture morphology presents ductile fracture characteristics. The above study provides a new method for improving the microstructure and mechanical properties of pipeline steel welds.