This study developed multifunctional sodium alginate-based hydrogel fertilizers (SHGBs) featuring freezeresistant, controlled-release, and biodegradable properties, achieved by integrating the amphiphilic permeation regulator, betaine, into the hydrogel matrix. After optimization, SHGB30 maintained structural integrity, macroscopic flexibility, and porous structure at -25 degrees C, with a freezing point depression of -31.0 degrees C (no freezing) and - 46.87 degrees C (pre-freezing), demonstrating significant freeze resistance. Its water absorption capacity reached 215.9 g/g, achieving 81.8% cumulative urea release within 72 h and complete release within 10 days, consistent with the Fickian diffusion model. Soil cultivation demonstrated that SHGB30 modulates pH and electrical conductivity, enhancing phosphorus and ammonium nitrogen availability alongside multiple enzyme activities. Under drought and freezing stress, 0.3 wt% SHGB30 significantly improved germination rate, biomass, and photosynthetic performance in Elymus nutans. These beneficial effects were closely associated with enhanced antioxidant enzyme activities, reduced malondialdehyde (MDA) accumulation, and increased osmolyte synthesis, offering a novel strategy for green agriculture in cold-arid regions.