Abstract: Achieving excellent mechanical properties, swelling stability, and other characteristics simultaneously represents a core challenge and a hot research topic for bio-based composite hydrogel materials. In this study, a solution blending–ionic crosslinking method was employed to blend 6-triphenylmethyl-2, 3-hydroxypropyl cellulose (6T-HPC) with sodium alginate (SA). By adjusting the 6T-HPC loading (1, 2, and 3 g), 6T-HPC/SA composite gels were prepared, and their key properties—including microstructure, mechanical properties, swelling behavior, and thermal stability—were systematically characterized. 6T-HPC was synthesized by regionally selective modification of microcrystalline cellulose (MCC). By introducing a rigid, hydrophobic triphenylmethyl group at the C6 position and flexible, hydrophilic hydroxypropyl chains at the C2/C3 positions, a molecular structure combining rigidity and flexibility was constructed. SEM images of the composite gel cross-section reveal that the incorporation of 6T-HPC transforms the gel cross-section from a smooth plane into a rough, porous structure. Mechanical property tests indicate that when the 6T-HPC loading is 2 g, the tensile strength of the composite gel reaches 4.56 MPa, representing a 334% increase compared to pure SA gel (1.05 MPa). The results demonstrate that the 6T-HPC/SA composite gel, based on region-selective modification and constructed through molecular design and a “hydrophobic microdomains–hydrophilic network” biphasic structure, achieves a synergistic integration of mechanical reinforcement and functionalization, providing a new approach for developing polysaccharide-based functional materials that combine eco-friendliness with high performance.