Modified sensing membranes based on fluorescence quenching were prepared by the sol-gel meth-od, using formamide as the drying control chemical additive, tetraethoxysilane as the main material, Ru( phen )3 Cl2 as the indicator. The membrane with the optimum thickness of 20- 50μm is uniform and crack-free,in which the indicator has a very small leaking rate. The membrane is immersed in water for 50h, the membranesensing parameter M decreases by less than 5% . The fiber optic oxygen sensor with the sensing membrane has adetection limit of 5 × 10-6 M( ppm ), a response time of less than 30s, excellent reproducibility and stability.