In order to make up for the misjudgment of fish freshness by colorimetry due to the unclear color distinction, a naphthol-benzothiazole derivative (HBO) was synthesized by the condensation reaction of naphthol as a fluorophore with benzothiazole salt. HBO could recognize organic amines by colorimetric and fluorescent responses, and allow nondestructive detection of salmon freshness. HBO showed an obvious response for all 12 amines tested in an EtOH/H₂O (1:9, V/V) system, had a low limit of detection (0.4 μmol/L for spermine), and could image for spermine in living cells. The sensing gel HBO agarose gel (HBOAL) was prepared by loading the probe onto agarose gels. When salmon flesh is stored with HBOAL, the fish can be judged to be spoiled if HBOAL exhibits a light red color in daylight and a yellow color in ultraviolet light. HBOAL distinguished clearly among the three grades of salmon fish freshness with accurate evaluation results, and therefore could be used to monitor the freshness of salmon fish in real time.