Graphitic carbon nitride (g-C₃N₄) is widely used in the field of photocatalysis due to its unique two-dimensional planar structure and suitable energy band structure. However, it has some disadvantages such as fast recombination of the electron-hole, low visible-light utilization efficiency and poor dispersion in water, which hinder its application. In this study, the hydrogel prepared by sodium alginate was used as matrix to improve the dispersion of Ag@C₃N₄ composite in water, and at the same time enhanced the separation efficiency of photoelectron-holes pairs, thus improving its photocatalytic performance. Firstly, g-C₃N₄ was synthesized by thermal polymerization and then exfoliated into nanosheets by ultrasound. Then, Ag nanoparticles were deposited in situ on the surface of g-C₃N₄ by solution method to prepare Ag@C₃N₄. Finally, hydrogel loaded with Ag@C₃N₄ (SA/Ag@C₃N₄) was obtained by using calcium ion as crosslinker and sodium alginate (SA) as precursor. The morphology, microstructure and phase composition of the as-prepared photocatalyst were characterized. The as-prepared SA/Ag@C₃N₄ exhibited a 1.5 times higher photocatalytic degradation rate of methyl orange than that of Ag@C₃N₄. The catalytic mechanism was investigated by photoluminescence spectrum, time resolved photoluminescence spectrum and electron paramagnetic resonance spectrum. The results showed that the surface plasmon resonance effect of silver nanoparticles together with the porous structure and mass transfer channel of sodium alginate hydrogel plays a synergistic role in the enhancement of photocatalytic performance.