Three advanced flocculants were used to treat the biochemical treatment samples extracted from the landfill leachate. In order to explore the transformation characteristics of dissolved organic matter (DOM) in the process of coagulation, all the samples’ DOM from three stages were characterized by ultraviolet-visible spectra (UV-Vis), three dimensional fluorescence spectra (EEM) and Fourier transform ion cyclotron resonance electrospray mass spectrometry (ESI FT-ICR MS). UV-Vis spectra showed that the ratio of small molecules increased in water while both the humification degree and condensation degree reduced after coagulation. EEM suggested that the removal efficiency of protein and humic acid substances were very high after coagulation. FT-ICR MS indicated that there was a strong selectivity of flocculants for the removable DOM in different stages of coagulation. DOM in the final residual water was mainly smaller molecular weight, containing impurity atoms with low unsaturation, O/C<0.3, and H/C>1.5.