Sensitivity is an important index to judge the performance of receiving and calibration system，which would affect the image quality of space-borne microwave radiometer directly. In order to verify whether the space-borne radiometers meet the requirements of observation or not, hot and cold calibration methods are adopted to test the receiving and calibration system sensitivity; the factors affecting sensitivity of receiving and calibration system and the measurement accuracy are analyzed. The results prove that the sensitivity of receiving and calibration system is better than 0.86 K，and the accuracy is higher than 0.1 K for F frequency. The system sensitivity can be improved by optimizing the design of quasi-optical feed network and minimizing channel insertion loss.