In the paper, Fourier transform infrared (FTIR) spectroscopy was used to study the fruiting body of wild growing and cultivated Coprinus comatus and the cap of the mushroom before and after deliquescing into inky liquid. The results show that the infrared spectra of Coprinus comatus are mainly composed of the absorption bands of protein and polysaccharide. Remarkable differences are observed in the absorption bands of polysaccharide between the spectra of the wild growing and cultivated mushrooms, by which the wild growing and cultivated Coprinus comatus can be discriminated. It was also found that the absorption bands of polysaccharide become weaker obviously in the black-inky samples of deliquescing cap compared with the un-deliquescing cap, indicating that the polysaccharides in the cap of Coprinus comatus are transformed during the process of cap deliquescing into a black, inky liquid. The spectral results can offer useful information for a further study of Coprinus comatus.