Instrument-free chlorine dioxide (ClO2) preservative for fruit and vegetable has gained great attention due to its convenience and safety. In this study, a series of carboxymethyl chitosan (CMC) with citric acid (CA) substituents were synthesized, characterized, and further used to prepare a novel ClO2 slow-releasing preservative for longan. UV-Vis and FT-IR spectra revealed that CMC-CA#1-3 were successfully prepared. Further potentiometric titration showed that the mass ratios of CA grafted in CMC-CA#1-3 were 0.18:1, 0.42:1, and 0.42:1, respectively. The composition and concentration of ClO2 slow-releasing preservative were optimized, and the best formulation was as follows: NaClO2:CMC-CA#2:Na2SO4:starch = 3:2:1:1. The maximum ClO2 release time of this preservative reached >240 h at 5-25 °C, and the maximum release rate always occurred at 12-36 h. Longan treated with 0.15-1.2 g ClO2 preservative had significantly (p < 0.05) higher L* and a* values but lower respiration rate and total microbial colony counts than the CK group (0 g ClO2 preservative). After 17 days of storage, longan treated with 0.3 g ClO2 preservative had the highest L* value of 47.47 and lowest respiration rate of 34.42 mg·kg-1·h-1, showing the best pericarp color and pulp quality. This study provided a safe, effective, and simple solution for longan preservation.