In this study, a novel natural polysaccharide hydrogel was prepared using dialdehyde sodium carboxymethyl cellulose (DCMC) and arginine-grafted carboxymethyl chitosan (Arg-g-CMCS) as the matrix. The microstructure, elastic modulus, and loss modulus of the hydrogel were characterized using Fourier transform infrared spectroscopy (FTIR), a rotational rheometer, and scanning electron microscopy (SEM). Finally, the hydrogel was applied for the postharvest preservation of grapes. The results showed that the gelation time of the hydrogel with a DCMC to Arg-g-CMCS ratio of 1:3 (V/V) was 15.67 s, and it exhibited a uniform porous structure and suitable rheological properties, with an equilibrium swelling ratio reaching 4734% within 72 h. The hydrogel suppressed the respiratory intensity of grapes, thereby delaying quality deterioration caused by metabolic activities. The treatment effectively slowed the decline in fruit brightness (L* value), inhibited the decrease in a* value and the increase in b* value, reduced mass loss, delayed the decline in firmness, and maintained the stability of nutritional and functional components in grapes, including soluble solids, titratable acid, vitamin C, total phenols, and total flavonoids, thus demonstrating favorable preservation efficacy. Therefore, the DCMC/Arg-g-CMCS hydrogel can delay postharvest quality deterioration in grapes, showing promising application prospects in the environmentally friendly and high-efficiency preservation of fruits and vegetables.