Biomethanation may be the most favourable method for low carbon conversion of peat. In order to explore the reuse of peat methane fermentation residue to extract humic acid and realize the new process of peat methane fermentation coupled with humic acid extraction, it is necessary to analyze the influence of alkali liquor on the yield, purity and structure of humic acid from methane fermentation residue. This paper uses herbaceous peat as raw material to conduct sodium hydroxide pretreatment and methane fermentation. The residue is extracted from humic acid by alkali dissolution and acid precipitation and from potassium hydroxide, sodium hydroxide, lithium hydroxide, and sodium pyrophosphate, respectively. The humic acid samples were characterized by yield, purity, UV-Vis spectrum, infrared spectrum and fluorescence spectrum. The results showed that the yield, content and structure of humic acid were significantly affected by extraction with different extractants and anaerobic fermentation with sodium hydroxide pretreatment. The yield and purity of humic acid in the residue of anaerobic fermentation and blank group pretreated by lithium hydroxide extraction and sodium hydroxide were the highest, with a yield of 43.59% and 41.07% and purity of 70.4% and 70.6%, respectively. The content of hydroxyl, methyl, methylene and ether bonds in peat humic acid extracted by sodium pyrophosphate is higher, and the degree of aromatization is low; the extraction degree of aromatic carboxylic acid and humic acid is high, and the content of aromatic aldehyde and humic acid is high. The contents of hydroxyl, methyl, methylene and ether bonds of humic acid in anaerobic fermentation residue pretreated with sodium hydroxide decreased, carbonyl and benzene ring contents were higher, and the degree of aromatization increased. The study on the change characteristics of extracting peat humic acid from anaerobic fermentation residue shows that sodium hydroxide pretreatment anaerobic fermentation has a significant impact on the structure of extracting peat humic acid by alkali dissolution and acid precipitation, and the process of reusing peat anaerobic fermentation residue to extract humic acid is feasible.