Characteristics and hydrogen-oxygen isotopic compositions of halite fluid inclusions in the Hongze sag of the Subei Basin were analyzed to reconstruct the paleoenvironment during salt formation and investigate the supply mechanisms of salt-forming materials. The results reveal that primary halite fluid inclusions in the fourth member of the Funing Formation from SB18 exhibit two distinct types: cumulate and chevron crystals, frequently coexisting within the same strata, indicating an shallow water environment during deposition. The hydrogen-oxygen isotopic composition of these fluid inclusions shows significant deviation from the meteoric water line, clustering in the lower right quadrant. Combined with homogenization temperature data and sedimentary sequences, this pattern reflects intense evaporative conditions during the salt formation process. The sources of salt-forming materials in ancient salt lakes mainly include the weathering and leaching of surface rocks, residual water from marine transgressions, and the lateral infiltration of seawater. Furthermore, the basin's unique “high mountain-deep basin” topography, developed under specific tectonic conditions, provided favorable paleo-environmental settings and accumulation spaces for the concentration of salt-forming materials.