A biomass based hydrogel soybean dregs-Poly(acrylic acid) (SD-PAA) was synthesized under UV radiation while using agricultural waste soybean dregs. Maximum absorption of SD-PAA is 3587 g·g⁻¹ in distilled water and 302.0 g·g⁻¹ in 150 mM NaCl aqueous solution. Moreover, the influence of granularity, salt solution, and ions in the solutions on water absorption is systematically studied. Sensitivity sequence of the hydrogel to cations was K⁺ < Na⁺ < NH₄⁺ < Al³⁺ < Fe³⁺ < Mg²⁺ < Ca²⁺, and that to anions was PO₄³⁻ > SO₄²⁻ > Cl⁻. Moreover, the experimental results showed that SD-PAA water retention capability remained 37% after centrifugating for 60 min and 0.2% being dried at 60 °C for 70 h. Meanwhile, the swelling data agree well with the pseudo-second-order kinetic model and Fickian diffusion mechanism.