Terahertz time-domain spectroscopy is utilized to detect the spectra of 3,4-dichloroaniline and 2, 5-dichloroaniline isomers within the spectral range of 0.2 to 2 THz. There are absorption peaks at 1.00, 1.48 and 1.88 THz in the spectra of 3, 4-dichloroaniline, comparing with absorption peaks at 0.83, 1.04 and 1.17 THz in the spectra of 2,5-dichloroaniline. The intensity of the absorption peaks for the two isomers are also obviously different. DFT(Density Functional Theory) is adopted to simulate the theoretical spectrum of two isomers, which indicates the theoretical spectra and experimental spectra are in good consistency. The results reveal that terahertz time-domain spectroscopy can be applied in the fingerprint detection of 3, 4-dichloroaniline and 2, 5-dichloroaniline isomers. It provides a rapid and accurate method for the detection of toxic compounds which have obvious absorption peaks in terahertz band.