In this paper,the THz and infrared spectrum of Phosgen and Lewisite and Sarin are calculated by the Gaussian theory.The calculated results of the infrared spectrum of this three biochemical drugs suggested that the application of Gaussian in simulating drugs′s THz spectrum is of great significance for the THz detection.The THz spectrum of the three substances by Guassian will provide very important information for us in detectting biochemical drugs with THz wave.