Homocysteine is an amino acid related to metabolism in human vivo, which is closely related to cardiovascular disease, senile dementia, bone fracture, et al. Currently, the usual medical test methods for homocysteine include high performance liquid chromatography (HPLC), fluorescence polarization immunoassay (FPIA) and enzyme-linked immunosorbent assay (ELISA), which are time-consuming or expensive. In this paper, we first analyze the vibration and rotation of homocysteine molecules by using density functional theory, and then we ensure that the theoretical absorption peaks are located in the range of the terahertz spectrum. Then, based on the terahertz time-domain spectroscopy system, we measured the absorption spectrum of homocysteine under different concentrations. It is found that as the detection of the concentration, the terahertz results present higher accuracy than that of the laser Raman spectrum, which can be used as the reference for the evaluation of pathological stage. These results are of great significance for the exact and quick diagnosis of homocysteine-related diseases in clinical medicine.