An abandoned petroleum well which had been exploited for about twenty years in Songyuan city of Jilin Province, China, was selected to investigate the structural characteristics of soil humic acid (HA) under different petroleum contamination levels. Surface (0-20 cm) soil samples were collected at 0.5, 1.5, 3.5, 5.5 and 7.5 m depths from well head. HA were extracted with 0.1 mol/L NaOH and 0.1 mol/L Na4P2O7 and were characterized with differential thermal-thermogravimetric analysis (DTA-TGA) and synchrotron-based soft X-ray near-edge adsorption (C-1s NEXAFS) spectroscopy. With the decrease in the distance of soil from well head, the reaction heat value and weight loss at both moderate and high temperature exothermic peaks from the DTA-TGA of HA increased, and the heat ratio of the high to moderate temperature exothermic regions increased. The C-1s NEXAFS data suggested that the relative contents of aromatic C and carboxyl C increased, while that of aliphatic C and the ratio of aliphatic C to aromatic C decreased. These results showed that aliphaticity of HA decreased while aromaticity increased with the increase in petroleum content. The results obtained by C-1s NEXAFS spectroscopy were mainly corresponding with those obtained by chemical analysis, thermal analysis, FTIR and 13C CPMAS NMR spectroscopy, which indicated that C-1s NEXAFS had a potential in characterizing the structural changes of HA in soil with different petroleum contamination levels.