A oxygen sensor based on fluorescence quenching is reported, plastic optical fiber as sensor head and light transmit element for oxygen sensor are adopted. The sensor head is made into U shape. We use Dichlorotris (1,10-phenanthroline) ruthenium (Ⅱ) as fluorescence indictor. Sol-gel method is used to prepare the sensor material. We detect the fluorescence lifetime by using phase modulation technique. The effect of bending radius on sensitivity is measured, and we find that the system sensitivity will increase when the curve radius decrease. We also measure the relationship between oxygen concentration and fluorescence lifetime, and find that they are sub-linearity. Double fluorescence site model is put forward to explain this phenomenon.