A ratiometric oxygen sensor based on a platinum octaethylporphyrin (PtOEP)–coumarin 6 (C6)/poly (styrene-trifluoroethyl methacrylate) (poly (St-TFEMA)) film was developed for automatic dissolved oxygen (DO) detection. The oxygen-sensing film according to the dynamic quenching mechanism was prepared by embedding platinum octaethylporphyrin (PtOEP) and coumarin 6 (C6) in poly (styrene-trifluoroethyl methacrylate) (poly (St-TFEMA)). The optical parameter (OP) was defined as the ratio of the oxygen-insensitive fluorescence from C6 to the oxygen-sensitive phosphorescence from PtOEP. A calibration equation expressing the correlation between the OP values and DO content described by a linear function was obtained. A program based on the Labview software was developed for monitoring the real-time DO content automatically. The influence of the excitation intensity and fluctuation on the OP values and the direct luminescence signal (integration areas) was compared, verifying the strong anti-interference ability of the sensor. The detection limit of the sensor was determined to be 0.10 (1) mg/L. The switching response time and recovery time of the sensor were 0.4 and 1.3 s, respectively. Finally, the oxygen sensor was applied to the investigation of the kinetic process of the DO content variation, which revealed an exponential relationship with time.