An optical method for the detection of potassium ion based on the conformational change of aptamer and fluorescent probe thiazole orange（TO）was reported.Anti K+ aptamer was hybridized with its complementary strand which has a single base mutation and is shorter than aptamer.TO exhibited stronger fluorescence when it interacted with the double strand DNA mentioned above.The fluorescence emission intensity of TO decreased when double strand DNA was dehybridized due to the formation of G quadruplex structure and single strand DNA in the presence of K+.The influences of TO concentration,temperature and time on the sensing of K+ were investigated.Under the optimum reaction conditions,a good linear relationship was found between the fluorescence decreasing ratio（I0F-IF）/I0F and the potassium ion concentration in the range of 10×10-6-20×10-4 mol/L,and a sensitive detection limit of 33×10-7 mol/L was obtained.Contrast experiments with other ions such as Li+,Na+,Ca2+,Mg2+ and NH+4 showed that this method was label free,simple and economical,and could be used for the determination of K+ in potassium penicillin V tablets with good sensitivity and specificity for K+.The results obtained by this method were in agreement with those from atomic absorption spectrophotomertic analysis.