On the base of absorption spectra in the range from 0.2 to 2.6 THz of nine illicit drugs using terahertz time-domain spectroscopy (THz-TDS) technique, the THz absorption spectra of the nine different illicit drugs were identified successfully by back propagation (BP) neural networks. Firstly, absorption spectra of the nine illicit drugs, which were pretreated by second derivation, were used to train the BP neural network. Secondly, absorption spectra of the nine illicit drugs which were measured in different time, pretreated by second derivative too, were identified by BP neural network and the identification rate of 89% was achieved. The model of BP neural network was processed in Matlab. The results indicated that it is feasible to apply BP neural network model on the identification of illicit drugs, and providing an effective method in the secure inspection and identification for illicit drugs using THz-TDS technique.