The chemical differences of traditional Chinese medicine leech before and after processing were analyzed by FTIR and two-dimensional correlation infrared (2D-IR) spectroscopy.The result showed that the leech was high in protein, with characteristic peaks of amide Ⅰ, Ⅱ bands.Comparing the IR spectra of samples, the primary difference was that the characteristic peak of fresh leech was at 1 543 cm-1, while that of crude and processed leech was at 1 535 cm-1.A 2D-IR spectrum with heating perturbation was used to track the processing dynamics of leech.In the 2D-IR correlation spectra, fresh leech exhibited stronger automatic peaks of the amide Ⅰand Ⅱ bands than that of processed leech, which indicates that the protein components of the fresh leech were more sensitive to heat perturbation than the processed one.Moreover, the result of FTIR and 2D-IR correlation spectra validated that the 3-dimensional structure of protein was damaged and hydrogen bonds were broken after processing, which resulted in the inactivation of protein.The fatty acids and cholesterol components of leech were also oxidized in this process.