We investigate the terahertz characteristic absorption spectra of homocysteine. First, we use the density functional theory to analyze the vibration and rotation modes of homocysteine and ensure that the theoretical absorption peak is within the testing range of the terahertz spectroscopy system. Subsequently, the characteristic absorption spectra of homocysteine with different concentration are measured based on the terahertz time-domain spectroscopy system, which can be accurately performed using the linear fitting equation. These results exhibit considerable significance for the accurate and rapid diagnosis of homocysteine-related diseases in clinical medicine.