The clinical application of penicillin is very extensive, and it is extremely important for the authenticity identification of such drugs and the identification of drug content. In this paper, the self-built terahertz time-domain spectroscopy system is used to test and analyze the pure penicillin sodium and amoxicillin capsules from three different manufacturers. The absorption spectra of four drugs in the range of 0.2-1.4 THz and the obvious absorption peaks are obtained. At the same time, the changes of absorption peak intensity of pure penicillin sodium and amoxicillin with different contents under the same mass are tested, and the corresponding relationship between the mass and intensity is clearly obtained. Finally, the intensities of three amoxicillin drugs are put together for comparison, and the correspondence between the content and the intensity can be visually seen. Spectroscopic study results on pure penicillin sodium and amoxicillin show that terahertz spectroscopy can be used for qualitative identification and quantitative analysis of penicillin drugs, which is of great significance to the substance identification of penicillin drugs.