In response to the recent negative news of several expired drugs, in order to further improve the speed of drug testing, this paper proposes a new method for nondestructive testing of drug quality using terahertz time-domain spectroscopy (THz-TDS). Firstly, the terahertz time-domain spectra of expired amoxicillin capsules, compound paracetamol and amantadine hydrochloride tablets, Banlangen and compound Banlangen granules were tested by the Zomega THz-TDS system (Z3), and the femtosecond laser produced by Mai Tai. Secondly, through the Fast Fourier Transform (FFT), the frequency spectrum of sample was obtained. Thirdly, according to the THz-TDS extraction optical parameter model proposed by Dorney and Duvillaret et al., the four cold medicines’ spectrum of terahertz characteristic absorption peak, absorption coefficient and the refractive index were obtained. Finally, the experimental results were compared with the terahertz characteristic absorption peaks, absorption coefficients and refractive indexes reported in the related literatures. Experiments show that the average refractive indexes of expired amoxicillin capsules and compound paracetamol and amantadine hydrochloride tablets in the effective frequency range of 0.2~0.9 THz are 1.90 and 1.85, respectively, which is smaller than the minimum refractive index reported in the related literature of 1.92 and 2.05, respectively. The average refractive index of expired Banlangen granules and compound Banlangen granules in the effective frequency range of 0.2~1.4 THz was 1.84, which was slightly larger than the maximum refractive index reported in the related literature 1.797. The five THz characteristic absorption peaks of expired amoxicillin capsules all appeared, but the amplitudes decreased. The three THz characteristic absorption peaks of compound paracetamol and amantadine hydrochloride tablets still exist, but the amplitudes are reduced, and a new terahertz characteristic absorption peak appears near 1.50 THz. The expired Banlangen granules have a new terahertz characteristic absorption peak at 1.43 THz and the absorption coefficient is reduced. The expired compound Banlangen granules appear three new terahertz characteristic absorption peaks at 1.43, 1.48 and 1.54 THz, and the absorption coefficient is also reduced. The results showed that the effective components of expired amoxicillin capsules, compound paracetamol and amantadine hydrochloride tablets, Banlangen granules and compound Banlangen granules were reduced, and some chemical components had been changed. The refraction index of western medicines such as expired amoxicillin capsules and compound aminophenol tablets will increase, while that of expired Banlangen granules and compound Banlangen granules will decrease. The above four kinds of expired drugs can be distinguished by the change of the refractive index and their terahertz characteristic absorption peaks, which could provide a reference for distinguishing expired drugs with terahertz spectroscopy.