<p>Lanthanum (La) was transferred into high-Mn austenitic cryogenic steel weld metal through submerged arc welding under a heat input of 15.74&#xa0;kJ cm⁻¹, utilizing an innovative method that involves the addition of La₂O₃ and silicon–calcium reducing agents to the flux. The evolution and formation mechanisms of types, number density, and size of inclusions in weld metals with La contents ranging from 0&#xa0;wt.% to 0.096 wt.% were investigated using scanning electron microscopy, focused ion beam scanning electron microscopy, transmission electron microscopy, and thermodynamic modeling. The results indicated that the addition of La can modify the irregular Al₂O₃ into spherical La-containing inclusions (LaAlO₃, La₂O₃ and La₂O₂S). As the La content increased from 0 wt.% to 0.096 wt.%, the inclusions underwent a stepwise evolution following the sequence Al₂O₃ → Al₁₁O₁₈La → LaAlO₃ → La₂O₃ → La₂O₂S, which accurately corresponds to the phase stability sequence predicted by thermodynamic calculations for the La-Al-O-S system. Simultaneously, the number density of the inclusions increased monotonically from 4745/mm² to 5893/mm², whereas the average size initially decreased from 0.56&#xa0;<i>μ</i>m to 0.42&#xa0;<i>μ</i>m, and then rising again to 0.49&#xa0;<i>μ</i>m. Specifically, the La-Al-O-S inclusions were extracted using an in-situ sample preparation technique, their internal structure and precise chemical composition were determined, and a comprehensive formation model of the La-Al-O-S system was established.</p>