Abstract: -lactam antibiotics are widely used in clinical practice, for their broad-spectrum, high efficiency and safety features. The identification and quality inspection of antibiotics are very important. Compared to other spectral methods, terahertz spectroscopy has some special advantages. In this paper, six kinds of antibiotics of clinical medication were tested using terahertz time domain spectroscopy(THz-TDS), and their terahertz absorption spectra were got. The results show that the samples have obviously different terahertz spectral characteristics even though they have similar molecular structures, which proves that the THz-TDS technology can identify small differences in molecular structure. So it can be very useful in medicament testing and analysis, which guarantees a good prospect in medicament development, production and identification. Finally, the absorption spectrum of benzylpenicillin sodium from 0.2-2.5 THz was simulated by Gaussian 03 software. Compared with the measurement results, the vibrational modes of the sample were assigned.