Singlet oxygen (¹O₂) is the main cytotoxic substance in Type II photodynamic therapy (PDT). The luminescence of ¹O₂ at 1270$$nm is extremely weak with a low quantum yield, making the direct detection of ¹O₂ at 1270$$nm very challenging. In this study, a set of highly sensitive optical fiber detection system is built up to detect the luminescence of photosensitized ¹O₂. We use this system to test the luminescence characteristics of ¹O₂ in pig skin tissue ex vivo and mouse auricle skin in vivo. The experimental results show that the designed system can quantitatively detect photosensitized ¹O₂ luminescence. The ¹O₂ luminescence signal at 1270$$nm is successfully detected in pig skin ex vivo. Compared with RB in an aqueous solution, the lifetime of ¹O₂ increases to $17.4\pm 1.2\mu$s in pig skin tissue ex vivo. Experiments on living mice suggest that an enhancement of ¹O₂ intensity with the increase of the TMPyP concentration. When the dose is 25$$mg/kg, the vasoconstriction can reach more than 80%. The results of this study hold the potential application for clinical PDT dose monitoring using an optical fiber detection system.Singlet oxygen (¹O₂) is the main cytotoxic substance in Type II photodynamic therapy (PDT). The luminescence of ¹O₂ at 1270$$nm is extremely weak with a low quantum yield, making the direct detection of ¹O₂ at 1270$$nm very challenging. In this study, a set of highly sensitive optical fiber detection system is built up to detect the luminescence of photosensitized ¹O₂. We use this system to test the luminescence characteristics of ¹O₂ in pig skin tissue ex vivo and mouse auricle skin in vivo. The experimental results show that the designed system can quantitatively detect photosensitized ¹O₂ luminescence. The ¹O₂ luminescence signal at 1270$$nm is successfully detected in pig skin ex vivo. Compared with RB in an aqueous solution, the lifetime of ¹O₂ increases to $17.4\pm 1.2\mu$s in pig skin tissue ex vivo. Experiments on living mice suggest that an enhancement of ¹O₂ intensity with the increase of the TMPyP concentration. When the dose is 25$$mg/kg, the vasoconstriction can reach more than 80%. The results of this study hold the potential application for clinical PDT dose monitoring using an optical fiber detection system.