Abstract: To address the issue of the poor mechanical properties and limited functionality of sodium alginate (Alg) gels, a composite hydrogel SCAP/Alg/Ca²⁺ that uses Alg and Schizophyllum commune alkali-soluble polysaccharide (SCAP) as substrates was prepared by induction of Ca²⁺. The gelation mechanism and the effects of SCAP addition amounts on the characteristics and antioxidant activity of the composite hydrogel were investigated. The results showed that the SCAP/Alg/Ca²⁺ composite hydrogel could be formed by the interactions of ionic bonds, hydrogen bonds and hydrophobic force. The composite hydrogel exhibited a highly ordered porous network structure with the addition of SCAP and Ca²⁺. When the SCAP addition amount was 3%, the water holding capacity of the composite hydrogel achieved 87.32%. At pH7.4, its swelling ratios were 45.56% (wet) and 2479.82% (freeze-dried), respectively. Rheological analysis revealed a significant enhancement in the storage modulus, increasing from 455.33 Pa for pure Alg hydrogel to 17768.56 Pa for the composite hydrogel, indicating improved rheological properties. The incorporation of SCAP and Ca²⁺ did not affect the thermal stability of the composite hydrogel but effectively enhanced its antioxidant capacity. The scavenging rate of DPPH free radicals increased from 6.95% for pure Alg to 26.65%, while the scavenging rates of ABTS⁺ and hydroxyl free radicals also improved by approximately 10%. Furthermore, the SCAP/Alg/Ca²⁺ composite hydrogel may also exhibit pH-responsiveness, maintaining structural integrity in gastric fluid while undergoing rapid degradation in intestinal fluid. The findings of this study provide a scientific basis and theoretical foundation for the biomedical carrier application of SCAP/Alg/Ca²⁺ composite hydrogels.