A fiber optic sensor for DNA detection based on fluorescence quenching was fabricated，using ［Ru（bipy）2DAFND］（ClO4）2 （Ru-DAFND，bipy=2，2′-bipyridine，DAFND=4，5-diazafluorene-9-p-nitrophenylhydrazone） embedded in sol-gel membrane as indicator for the optic sensing membrane.The membrane fixed on the sensor head was prepared by ethyl silicate hydrolysis using Ru-DAFND as fluorescence indicator，formamide as drying agent，and HCl as catalyst.The sensor was designed by using lock-in amplifier for signal measurement to detect ctDNA concentration.The results indicated that a linear relationship between relative phase delay （Δφ） of sensing membrane and DNA concentration was observed in the range of 2.6×10-8-5.4×10-6 mol/L in Tris buffer （pH 7.1），with a detection limit of 8.4×10-9 mol/L.The response time of senor was 70 s.The effects of solution pH value and foreign interferences to the performance of the sensor were investigated,and the results showed good stability and repeatability.