Abstract: The recombination action between organic polymer and nano TiO₂ is an important way to promote the photocatalytic property of composite materials. The nano carboxylic chitosan (Nano-CCS) was prepared by the selective oxidation of chitosan and ionic gel method, and then the composite nanoparticles of CCS/TiO₂ were obtained through the composite reaction of Nano-CCS with anatase TiO₂. The microscopic morphology, chemical structure and photoelectric properties of composite nanoparticles were characterized by SEM, DLS, BET, XRD, photoluminescence (PL) spectra, EIS and EPR technologies, the photodegradation property and catalytic degradation mechanism of methylene blue (MB) by composite nanoparticles were respectively analyzed. The results showed that the composite nanoparticles size was about 181-226 nm, with large specific surface area and a low band-gap energy. FTIR displayed that the composite nanoparticles contained the carboxyl groups and positive amino groups, which could attract the holes and electrons on TiO₂, and prevent the quick recombination of holes-electrons, so further promoted the generation of active oxygen, moreover the synergistic effect of adsorption and active oxygen by the composite nanoparticles accelerated the photocatalytic degradation of MB dye. The scavenger experiment also revealed that superoxide radical (·O₂⁻) was the important active substance during the photodegradation of MB dye solution under ultraviolet light, whereas other active oxygen (·OH and h⁺) played an auxiliary role. This research can provide the theoretical basis and practical application for developing the efficient photocatalysts and realizing photodegradation mechanism of dyes.