Abstract: Combining with the characteristics of high yield of mineral humic acid (HA) and high activity of biochemical HA, co-thermal oxidation of low rank coal and biomass to produce complex HA (MIXHA) was newly proposed. The mixture (MIX) of Heilongjiang lignite (HL) and wheat straw (WS) was co-thermally oxidized in 10% HNO₃ solution to prepare MIXHA. This work focused on comparison of the macro morphology and microstructure of MIXHA between HLHA and WSHA by SEM, FT-IR and ¹³C NMR analyses, and explored the synergistic effect between HL and WS during the co-thermal oxidation process. The results show that MIXHA content is higher than the theoretical value. Decomposition of HNO₃ molecular produces active oxygen atoms and nitrogen oxides to attack the molecular structure of WS and HL. Due to hydrogen bond rearrangement, glycosidic bond rupture, and crosslinking, plenty of alkyl radicals generated in WS are combined with the condensation aromatic ring in HL. Thus, the protonated aromatic carbon is changed into aliphatic substituted aromatic carbon. The obtained MIXHA is rich in oxygen-containing functional groups, and has high activity. Obvious characteristic peaks are observed in FTIR spectra of MIXHA. This work would provide a new idea for classification and resource utilization of low-rank coal and agricultural and forestry wastes.