By using Ru(phen) 3Br 2 as the indicator, a fiber optic oxygen sensor based on fluorescence quenching has been developed. The weak fluorescence signal has been detected by the lock in technology and the detection of the concentration of oxygen has been achieved by detecting the hysteresis of phase shift. The sensor has the detecting limit of 5×10 -6 and the detecting accuracy of 5×10 -7 , and its response time is less than 10 seconds. It also has good reproducibility, stability and strong ability of immunity from disturbance.