||Water-soluble conjugated polymers have an effective ability of signal amplification compared with corresponding monomer and can be dissolved in aqueous solution. Using their unique optoelectronic properties, in polymer structure, introduction of functional groups that can bind to specific target materials enables to apply chemo- and bio detection of DNA, protein, and biological agents. For this reason we designed fluorescent water-soluble conjugated polymer containing thymine and water-soluble sulfonate groups in the side chains, and benzothiadiazole units in the main chain. Because thymine groups can be interacted with mercury ion, the polymer structure has a selective detection of mercury ion. Detection of mercury ions was estimated fluorescence color change by exciton migration intra- and intermolecularly.