To detect the large-scale vessel targets in the whole day,all-weather and concealed,the syn-thesis aperture microwave radiometers with high resolution are proposed for the first time as a space-borne microwave thermal radiation passive detection system,whichis used to detect the vessel targets on the sea. A novel methodis proposedto quantitatively evaluate the detection capability of aspace-borne microwave passive detection system. The relationship between system’s main specifications, the microwave radiation cross section ofthe target and the detection distance is established. The detec-tion probability and the detection distance are derived,respectively. Simulation analysis and air-borne experiments are also carried out.Both simulation and experimental results demonstrate the feasibility of the space-borne synthesis aperture microwave thermal radiation passive detection technology with high resolution to detect the large-scale vessel targets. The synthesis aperture microwave radiometers with high resolution can be used as an important complementary system for space-based warning.