Terahertz spectroscopy can provide the lattice vibration information of solid materials, which is different from previous measuring methods. And particularly, it is suitable to test polymorphs. Thus it has an important meaning for controlling and detecting the polymorphs of drugs. The polymorphs should affect the properties, such as stability,of the drugs in storage. Nalidixic acid, the earliest quinolone antibacterials used, was investigated in this work. Two polymorphs NA-Ⅰ and NA-Ⅱ were synthesized and were confirmed by X-ray powder diffraction experiments. Different absorption spectra in the range of 0.2~2.4 THz of two synthesized polymorphs of nalidixic acid were obtained with terahertz time-domain spectroscopy at room temperature. Significant differences existed in the terahertz spectra of NA-Ⅰ and NA-Ⅱ. Five characteristic absorption peaks observed in the terahertz spectrum of NA-Ⅰ were at 0.94, 1.41, 1.88, 2.05, and 2.16 THz, while six peaks of NA-Ⅱ were at 0.72, 0.96, 1.38, 1.80, 2.04 and 2.16 THz. The most obvious difference was that there was a shoulder peak at 2.04 THz for NA-Ⅱ, while the intensity of the peak at 2.05 THz for NA-Ⅱ was much larger. Unlike other spectroscopies, such as infrared spectroscopy, the characteristic peaks in terahertz spectra were not related to specific functional groups. Thus the assignment of the characteristic peaks in terahertz spectra was essential. Theoretical calculations for the two polymorphs were performed based on density functional theory. The calculated characteristic peaks were in accord with those in the experiments. The vibrational modes in this low frequency region were mainly skeletal vibrations. Also the difference of the two spectra was explained by the different packing modes of the two crystals. This work provides evidence for identification of drug polymorphs using terahertz spectroscopy.