A terahertz (THz) imaging system and the image formation is based on the theory of compressed sensing (CS) is described. CS combines sampling and compression into a single non-adaptive linear measurement process, and then reconstructs the original image by using measurements based on reconstruction algorithm. It is abtained that a single intensity value by measuring the inner of a single mask and original image and a series of measurements with the same number of masks. CS permits the reconstruction of a N pixel×N pixel image using much fewer than N2 measurements to reduce the imaging time. This approach eliminates the need for raster scanning of the object or the THz beam, while maintaining the high sensitivity of a single-element detector. The experiment using a backward wave oscillator (BWO) which is a continuous-wave THz source and get a preliminary test result.