Abstract: Terahertz techniques is a hot point in the electromagnetic areas recently, many literatures have been published. In this paper, a problem of terahertz signal transmission and connection of terahertz components that involves the building of terahertz system is discussed. Quasi optical techniques is introduced to building terahertz systems, some examples are described to show the applications of the quasi optics in build terahertz systems including imaging system, power combining system and scanning antenna.