Water retaining agents (WRAs) are widely used in soil water retention, but their generally poor salt tolerance limits their practical application. This study proposes a "terpolymerization and dual-filler synergy" enhancement strategy, using acrylic acid (AA), acrylamide (AM), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as monomers, sodium hexametaphosphate-modified bentonite (SHMP-BT) and corn stover biochar (BC) as composite fillers, to prepare a novel composite salt-tolerant water retaining agent (TBB-WRA). The results indicate that TBB-WRA successfully formed an "inorganic layered-porous carbon" multi-level network structure and exhibited excellent thermal stability. Performance testing highlighted its advantages in highly saline and alkaline environments: the water absorption capacity reached 68.1 g/g in a 20% high-concentration NaCl solution and 875.09 g/g in a strongly alkaline (pH 12) solution, with both core indicators significantly outperforming commercially available WRA. Furthermore, TBB-WRA exhibits water absorption rates of 2055.51 g/g and 145.77 g/g in deionized water and 0.9% NaCl solution, respectively, combining rapid water absorption (reaching 66.5% of equilibrium swelling in 5 min) with long-lasting water retention (retaining water for up to 240 h). Soil simulation experiments indicate that adding 1%TBB-WRA can increase the maximum water-holding capacity of saline-alkali soil by 62% and extend the water retention time by 85.7%, demonstrating its potential for application in soil remediation of saline-alkali lands in arid regions.