The Ross Sea is one of the key regions for the generation of High Salinity Shelf Water (HSSW), and changes in this water mass exert a substantial influence on the formation of Antarctic Bottom Water (AABW). This study uses measurements of the oxygen isotope composition (δ¹⁸O) of seawater in the Ross Sea during the summer and autumn of 2025 to quantitatively evaluate the contributions of sea ice meltwater (SIM), meteoric water (MW), Circumpolar Deep Water (CDW), and HSSW. A Bayesian isotope mixing model is applied to assess these contributions and thereby to elucidate how freshwater input affects the formation of HSSW in the Terra Nova Bay polynya (TNBP). Our results show that HSSW is formed primarily from modified Circumpolar Deep Water (mCDW) through sea ice formation and associated brine rejection. MW plays a dual role in HSSW formation, providing both thermodynamic enhancement and dynamical inhibition. Thermodynamically, MW input cools the surface layer and increases the freezing point of seawater, both of which favor sea ice formation. Dynamically, however, MW input strengthens water column stratification, suppressing the vertical transport of brine and freshening the surface layer, thereby limiting the vertical development of HSSW. Within the Drygalski Trough, the mean HSSW fraction in the water column decreases from south to north, whereas the CDW fraction increases in the same direction, indicating a circulation pattern characterized by eastward transport of CDW and westward transport of HSSW. At the mouth of the Drygalski Trough, the δ¹⁸O–salinity (S) relationships of four water masses—HSSW, mCDW, pre-AABW, and AABW—exhibit a clear linear mixing trend. Based on this relationship, we estimate that a decrease of 0.05 in HSSW salinity induced by MW input would reduce the salinity of the resulting AABW by approximately 0.035, highlighting the critical role of freshwater in modulating AABW formation through its impact on HSSW. Under future warming in the West Antarctic, enhanced ice-shelf melt is likely to further weaken the formation of AABW.