In this study, conventional fluorescence spectroscopy in the excitation, emission and synchronous scan modes and three-dimensional fluorescence spectroscopy in the form of excitation-emission matrix of fluorescence intensity as a function of excitation and emission wavelengths were applied to study the complexation between DOM extracted from landfill leachates and Hg(Ⅱ) ions. The emission spectrum of DOM exhibited a broad peak with a center at 425 nm and a disorder change of the peak with increasing Hg(Ⅱ) concentrations, which suggested that the structure of DOM was comparatively simple and the fluorescence character of DOM-Hg(Ⅱ) complexes resulted from interaction of all fluorescence groups. The excitation spectrum of DOM showed that the intensities of two peaks at 392 and 458 nm both decreased with the addition of Hg(Ⅱ), indicating that different sources, hydroxy and amido groups, were all involved in the DOM-Hg(Ⅱ) complexation process. Synchronous-scan excitation spectra of DOM-Hg(Ⅱ) complexation showed that Hg(Ⅱ) not only produced fluorescence quenching effect, but also enhanced the rigid structure of DOM at a low concentration. The three-dimensional fluorescence spectra of DOM-Hg(Ⅱ) showed that the peaks A and B reduced strongly and the two peaks tended to shift toward longer wavelength with the concentrations of Hg(Ⅱ) increasing. These results indicated that protein-like matter reacted with Hg(Ⅱ) and there was a charge-transfer transition either between energy level in its ligand and a mercury energy level or between two mercury energy levels at the same time. Besides, the decrease in fluorescence intensity of peaks C and D in three-dimensional fluorescence spectra suggested that carbonyl and carboxyl formed bonds with Hg(Ⅱ) when DOM was complexed with Hg(Ⅱ).