Effects of different liquid acid precipitation on the spectral characteristics of humic acid extracted by alkali solution acid precipitation method were studied to select the preferable liquid acid, and analyze the mechanism of new process of peat combined production by methane fermentation and humic acid extraction. Used the peat for methane fermentation, then adopted the alkali extraction acid precipitation method by different liquid acids to extract humic acid from fermented peat residue and unfermented peat. Finally, the humic acid was characterized by Fourier transform infrared spectroscopy analysis, fluorescence spectrum analysis and E4/E6 analysis. The results of extracting peat humic acid showed that the yield of humic acid the PHA1 was the highest, the PHA2 was somewhat lower. The yields of humic acid from PHA1, PHA2 precipitated by nitric acid were 45.30%, 35.00%. And the purity of humic acid precipitated by nitric acid was higher, which in PHA1 was 54.83 mg·L-1, and in PHA2 was 61.03 mg·L-1. Considering the yield and purity, nitric acid was the best liquid acid for extracting peat humic acid by alkali-extraction acid-precipitation method. Fourier transform infrared spectroscopy analysis showed that the humic acid by nitric acid precipitation contained the most O—H groups, and there were more fatty carbon chain structure, alkyl and alcohol hydroxyl groups. The infrared spectra of humic acids obtained from PHA1 and PHA2 were similar. There was little difference between the triple bond,the cumulative double bond and hydroxyl groups of PHA1 and PHA2. The fluorescence spectrum analysis showed that the humic acid from peat had a peak value of about 450 nm, the peak value of PHA1 humic acid was the highest, and that of the PHA2 was the lowest. The peak value of phosphate acid precipitation humic acid was the highest, followed by nitric acid, indicating that the number of humic acid functional groups obtained by different liquid acid precipitation was different. The results of E4/E6 analysis showed that the E4/E6 ratio of PHA1 humic acid was higher and the degree of aromatic condensation was lower. After fermentation, the E4/E6 of PHA2 decreased, the degree of aromatic condensation was higher. Therefore, methane fermentation could consume more humic acid with low aromatic condensation degree, but the complex aromatic structure could not be degraded, and the aromatic condensation degree of extracted humic acid was obviously increased. Study on the characteristics of the spectroscopic changes in the extraction of peat humic acid by alkali-extraction acid-precipitation method showed that different liquid acids and methane fermentation have obvious effects on the yield, purity and functional groups of peat humic acid by alkali-extraction acid-precipitation method. The process of co-production of methane and humic acid from peat is feasible.