A new method for the study of triplex DNA is established according the fluorescence enhancement of molecular "Light Switch" complex of Ru(phen)2(dppx)2+ when it intercalate into triplex DNA. Because the fluorescence intensity of Ru(phen)2(dppx)2+ bonded to triplex DNA is in the case higher than that bonded to duplex DNA in certain range of DNA concentration, the method is much more sensitive than other methods reported previously.