Specific oligonucleotides such as telomere DNA and aptamer often undergo eonformational changes upon ligand binding. Composite reagent composed of o-phthalaldehyde and β-mercaptoethanol(OPAME) has been exten-sively applied to fluorescent detection of amino compounds based on the reaction of primary amino-group, herein we proposed a general spectrofluorometry for ions and small molecules due to conformational changes upon ligand binding taking K+ and ATP as examples. In a borate controlled buffer medium, telomere DNA could react with OPAME, giving a thio-subtituted isoindole compound with strong fluorescence emission at 455 nm when excited at 340 nm. It was found that however, the fluorescence emission was greatly reduced in the presence of K+ since the formation of the quadruplex structure inhibits the reaction activity of amino-groups of telomere DNA. In order to tes-tify the general application of OPAME reagent based on the eonformational change of oligonucleotides, we further proposed a sensitive method of ATP based on its highly selective interaction with ATP-aptamer. The above mentioned applications show that the speetrofluorometry with the aid of OPAME reagent is simple, label free that is expected to be potentially general for DNA conformational change-based target detection.