Humic acid-like substances (HALS) are heterogeneous macromolecules having a range of physicochemical properties and exhibiting different reactivities in environmental systems. However, to study the HALS that were isolated from atmospheric falling dust is seldom compare to which were extracted from soils and sediments. To better understand the chemical composition and heterogeneous property of HALS, which were obtained in the present work by multiple extraction of atmospheric falling dust, and their chemical composition was characterized using pyrolytic gas chromatography mass spectrometry (Py-GC-MS). This quick and effective procedure provides an insight into the structure and source of HALS. The results indicated that polysaccharides, lignin, protein/amino acid and aromatics are the main sources of this macromolecule organic fraction. Higher content of aromatic compounds which reached 67.42% suggested that aromatic structure is an important unit for HALS. Lignin-derived aromatic compounds are also major components of pyrolysates, while compounds derived from proteins and carbohydrates at higher concentrations were also detected. Compared with HALS fraction from dust in different sampling sites, it was suggested that biological emission and anthropogenic activity were the main contributors in suburban area of the university town and anthropogenic pollution was the main sources in urban area of the Wushan. The relative contents of pyrolysates in different seasons at the same sampling site indicated that meteorology is also an important factor to influence composition of HALS.