The ultrafast excited-state dynamics of a cyanine dye, 3,3'-bis(3-sulfopropyl)-5,5'-dimethoxy-thiacyanine triethylaminium salt, was investigated by using conventional time-resolved fluorescence up-conversion technique. The fluorescence decay can be well described as tri-exponential kinetics, which indicates the excited-state population decays through the bond-twist, vibrational and radiative relaxation channels. Further analysis shows that the contributions of the three relaxation channels to the fluorescence decay demonstrate very different change with increasing the fluorescence wavelength, through which the detailed dynamics at different regions in the excited-state potential energy surface can be retrieved.