Chitosan is non-toxic, harmless, biocompatible, and antimicrobial, and can be readily modified. These properties have made it widely used in carrier research. Based on this, a fluorescent probe P was synthesized in high yield from naphthalimide derivatives and carboxymethyl chitosan (CMCS). The probe exhibited enhanced fluorescence in the presence of Al³⁺ and quenched fluorescence in the presence of Mg²⁺ in different media. Among common metal ions and anions, the probe demonstrated good selectivity and sensitivity toward Al³⁺ and Mg²⁺. Under optimal conditions (ethanol–water solution, 1:9, v:v, pH 6.0, 20 mM HEPES), a significant linear relationship was observed for Al³⁺ in the concentration range of 0–90 µM. In ethanol, the fluorescence intensity of the probe at 427 nm decreased regularly with increasing Mg²⁺ concentration, also showing a clear linear relationship within the 5–90 µM range.