A continuous-wave (CW) terahertz (THz) standoff scanning imaging system at 0.2 THz is presented. This system works at reflection geometry and the imaging distance is 25 m. A Gunn oscillator is utilized as emitter and an unbiased Schottky diode operated at room temperature is employed as detector. A polyethylene Fresnel lens is used to collimation terahertz wave for standoff propagation. Six aluminum mirrors are employed to increase distance. The sample is placed on an X-Y two-dimensional stage which is controlled by a computer. The collimated THz wave propagates in air and is focused to the sample by another polyethylene Fresnel lens. The back scattered THz wave from the sample surface is collected by the detector alone the same path. The two-dimensional image of sample is obtained by a raster scanning fashion. An aluminum plate with holes, an airplane model and a toy gun contained in a box are imaged at 25 m from the imaging unit. The results show that this standoff imaging system has a wide potential to be applied in the area of security inspection and screening.