【Objective】Cyanobacteria are the oldest photoautotrophic organisms on the earth. As they lived commonly in the ancestral protohydrosphere and entered into the geosphere with geological changes, they got profoundly involved in the formation of primitive soil humic substances (HS). Since contain large volumes of protein, polysaccharide and even some humic-like acid substances in their cells, extra- and intra-cellular metabolites, is it possible for cyanobacteria cells to form HS spontaneously? If the formation process needs catalysis of some microorganisms, what are the differences between different groups of microorganisms in role in the process? These scientific problems need to be solved. 【Method】 In view of the above-mentioned scientific problems, this research adopted the source control and shake-flask liquid axenic culture method to get axenically cultured samples of Anabaena flos-aquae，a representative species of cyanobacteria, in different growth stage and products of Anabaena flos-aquae used as the only substrate and treated with different microbes. The products were analyzed for structural characteristics, using the method of element composition and fourier transform infrared spectroscopy (FTIR), with a view to exploring possibility of cyanobacteria spontaneously forming HS or acting as substrate in HS formation and hence to make up and specifying the role of cyanobacteria in the origination and formation of soil HS. 【Result】The following findings were obtained. Under the experimental conditions, Anabaena flos-aquae had a long life cycle and remained quite constant in carbon content during their growing period. However, during the logarithmic phase, the carbon content in the thallus was relatively high, while in the extracellular metabolites it rose steadily but remained much lower than that in the thallus. None of the FTIR spectra of the cyanobacteria samples showed any peaks characteristic of the carboxyl C=O bonds at 1 720 cm^-1, instead, the spectra intensified somewhat at 1 650 cm^-1 and 1 530 cm^-1. Relative to soil HS, cyanobacteria thallus were rich in organic nitrogen, but poor in organic carbon and high in protein and components of polysaccharide. The extracellular metabolites of cyanobacteria contained a large amount of hydroxy and carboxyl functional groups. The ratio of 2 926/1 650 revealed that during the logarithmic period and the late decline phase, Anabaena flos-aquae thallus contained a high amount of aromatic substances, which were quite approximate to soil fulvic acid (FA), but lower than soil humic acid (HA) in complexity. Relative to soil humus, Anabaena flos-aquae thallus were high in both H/C ratio and (O+S)/C ratio, which means that they were low in condensation degree and high in oxidation degree. Although it was found that the possibility of cyanobacteria spontaneously forming HS was very low, it could not be ruled out that under extreme conditions (abnormal temperature or pressure) cyanobacteria might get combined with other polymers, forming HS, or form HS by the action of microbes as during their long life cycle some organic substances complex in structure may be produced. Under aerobic conditions, Anabaena flos-aquae was used as the only carbon source and treated with Bacillus megaterium, Sreptomyces sp. and Trichoderma viride and the resultant thallus mixture contained more aromatic substances, but the peak at 1 530 cm^-1 disappeared in the FTIR spectra of the thallus mixtures, which indicates that part of the C=C bonds and amino acid-like materials in the mixture were decomposed by microorganisms. The thallus mixture was lower in aromaticity, higher in oxidation degree and simpler in structure, which indicates that once the cyanobacteria culture was inoculated with aerobic microbes, the microbes first made use of the sugar components in the culture for cell proliferation first, and then began to play their limited role in decomposing cyanobacteria cells. The Van-Krevelen plot shows that the thallus mixture formed after the treatment with aerobic microbes differed more sharply from soil HS in structure. By comparing how much they differed, it was learnt that actinomyces and bacteria were more capable of decomposing cyanobacteria, thus simplifying the thallus mixture in structure, while fungi made the thallus mixture of cyanobacteria closer to soil FA and more complicated in the structure.【Conclusion】In a word, under the lab conditions, the thallus mixture formed by cyanobacteria per se and/or using cyanobacteria as the only substrate does differ from soil HS, but the thallus mixture, thanks to its composition and structure, may serve as substrate for HS formation. The findings may provide a certain scientific basis for the research on formation and origin of soil HS.