Abstract: The development of Terahertz technology and research progress is briefly summarized. The unique properties of Terahertz wave, such as low energy, high penetrability, dactylogram, broadband, transient and coherent are generally introduced. Existing and emerging applications of terahertz technology in imaging, radar, communication, biomedicine, antiterrorism and security detection, spaceflight, astronomy and geography are illustrated. An emphasis is put on the field of radar detection and communication system. For high range resolution, ultrawide band, powerful penetration, low probability of intercept and robust jamming immunity, antistealth and penetration plasma capability, THz radar is able to actualize various function of the detection system. THz communication has broad application prospects. THz wave has wide and unexplored region of electromagnetic spectrum. THz wave has good characteristics of high speed, directivity, security, scattering, penetration etc. THz communication has great promotion effect to the traditional communication system. The application principle and key technology of THz technology to radar detection and communication system are presented in detail.