To detect the large-scale vessel targets in the whole day, all-weather and concealed, the synthesis aperture microwave radiometers with high resolution are proposed for the first time as a space-borne microwave thermal radiation passive detection system, which is used to detect the vessel targets on the sea. A novel method is proposed to quantitatively evaluate the detection capability of a space-borne microwave passive detection system. The relationship between system’s main specifications, the microwave radiation cross section of the target and the detection distance is established. The detection probability and the detection distance are derived, respectively. Simulation analysis and air-borne experiments are also carried out. Both simulation and experimentalresults 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.