The dye chemosensor for mercury ion detection was designed, which was based on donor-acceptor intramolecular charge transfer system and the corresponding fluorescence changing property was successfully monitored. This new dye chemosensor was containing dimethylcarbamodithioate groups to coordinate $Hg^{2+}$, which showed very effective sensing functions. But it showed no significant changes upon the addition of other metal ions such as $Ca^{2+}$, $Pb^{2+}$, $Al^{3+}$, $Ce^{2+}$, $Ba^{2+}$, $Ni^{2+}$, $Cd^{2+}$, $Zn^{2+}$, and $Mg^{2+}$.