Soil salinization severely threatens agricultural productivity in arid regions. This study comprehensively evaluates carboxymethyl cellulose (CMC) as an eco-friendly soil amendment through laboratory and a two-year field experiment. Laboratory results showed that CMC's swelling capacity was inhibited by high salinity but slightly enhanced at a low salt concentration (1 g & centerdot;kg-1 NaCl). CMC exhibited excellent biodegradability, which was moderately slowed in saline soil. A distinct dose-dependent effect on maize was observed: low concentration (0.1 g & centerdot;kg-1) promoted seedling growth (root length increased by 77%; stem length by 29%) and boosted nitrate reductase (NR) activity. In contrast, high concentrations (>= 0.5 g & centerdot;kg-1) induced severe stress, evidenced by elevated malondialdehyde (MDA) and proline levels. Crucially, CMC application reduced the soil water infiltration rate. The field trial confirmed this mechanism, showing that CMC retained irrigation water within the 0-40 cm root zone by impeding deep percolation. Consequently, compared with the control, the 300 and 450 kg & centerdot;ha-1 CMC treatments increased maize grain yield by up to 23.41% and enhanced water use efficiency (WUE) by up to 22.38%. Under the conditions of this study, CMC functioned primarily as a root-zone water regulator rather than solely as a water reservoir. By slowing water infiltration and reducing nonproductive water losses (evaporation and deep percolation), it optimizes the soil water balance for crop use. This study indicates that appropriate CMC application can improve water retention, crop performance, and water use efficiency under saline conditions.