Fish-based food products play important roles in our daily diet. The related food safety is vitally essential for human health, thus it is very necessary to screen the freshness of fish-based foods. In this work, we presented a ratiometric fluorescent probe PTCN for the determination of cadaverine, a metabolic biomarker of the spoilage of fish. PTCN displayed a ratiometric fluorescence response towards cadaverine with good specificity, high sensitivity (LOD = 46 nM) and ultra-fast response (<15 s), and thus has been successfully utilized to determine cadaverine from the spoilage of fish. PTCN was fabricated into cheap and portable sensing tags, which can visually detect gaseous cadaverine with obvious fluorescence color transformation from red to green and a low detection limit (8.65 ppm). Moreover, the PTCN tags were used as smart fluorescent tags for non-contact and visual monitoring of cadaverine in fish. Furthermore, the ratiometric fluorescence signals were utilized to create a smartphone-adaptable digital sensing profile for indicating cadaverine in fish products.