Terahertz radiation imaging enables passive target detection. To accurately obtain terahertz radiation images of targets and the background environment, a terahertz radiation imaging system is designed. The system performs spatial sampling of the field-of-view through two-dimensional scanning, employs a 0.5 m reflector antenna to collect radiation signals into a radiometer with a center frequency of 0.15 THz, and then reconstructs the radiation image of the field-of-view by combining the voltage signal output from the radiometer with spatial sampling parameters. To investigate ground target detectability, visible light, 0.15 THz, and infrared images of the field-of view are measured under day/night and rainy/foggy conditions. Experimental results demonstrate that the 0.15 THz radiation imaging system can capture radiation energy from the cosmic microwave background, enabling accurate detection of high-reflectivity targets during both day and night, as well as in rainy conditions.