Coal gasification process generates a huge amount of highly concentrated industrial wastewater. A better understanding of molecular compositions of dissolved organic matter (DOM) is necessary for the design and optimization of coal gasification wastewater (CGW) treatment processes. In this study, we analyzed the chemical compositions of DOM in a Lurgi coal gasification wastewater treatment plant by three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry. Six samples were collected from several specific treatment processes, including raw CGW, effluent of hydrolytic acidification, effluent of sequencing batch reactor, effluent of contact oxidation, and influent and effluent of NaClO bleaching process. The CGW DOM was separated into organic and aqueous phases using liquid-liquid extraction (hydrophobic part) and following solid-phase extraction (hydrophilic part). The concentration of DOM decreased along the whole process, whereas its hydrophilicity increased. The 3D-EEM results reveal that three distinct fluorophores: protein-like organic matter, fulvic acid-like organic matter, and humic acid-like organic matter exist in raw CGW. The intensity of the three fluorophores significantly reduced along these treatment processes, and only fulvic acid-like organic matter can be identified in the end effluent. The molecular compositions of DOM in organic and aqueous phase show different patterns; CHO formulas were the dominant species in the organic phase, where CHOS formulas were predominant in the aqueous phase. The chemical diversity of DOM increased with different treatment processes; especially, potential disinfection by-products like CHOCl compounds are formed during NaClO bleaching process. Semiquantitative analysis using stearic-d(35) acid (C18D35O2H) revealed the decreasing trend of different species, providing detailed information of molecular transformation in the water treatment processes.