Due to the ageing of the hydrophobic layer of the radome, the ground-based microwave radiometer is prone to form water accumulation on the radome during rainy weather detection, resulting in unstable detection performance, which is one of the key problems affecting its wide application in scientific research and business. This study uses the actual observation data of 6 microwave radiometers in Beijing during the rainfall period from 2017 to 2018. It analyzes the influence of the ageing of the hydrophobic layer of the radome on the observed brightness temperature of the microwave radiometer. The results show that increasing with the use time, especially after three months, the magnitude of the increase in the brightness temperature during precipitation becomes significantly larger, and the duration of the increase in the brightness temperature after the precipitation also increases. At the same time, the rain test of the radome is carried out using the microwave radiometer in Beijing Nanjiao Station. The results show that after the radome is used for 40 days, with the pollution and ageing of the radome, the observed brightness temperature increases in a jumping manner during rain, and the brightness temperature is still high after the rain, even after more than 2 hours. Therefore, in order to improve the availability of microwave radiometer observation data, it is recommended to replace the radome every quarter during actual use. For heavily polluted areas such as Beijing, Tianjin and Hebei, the replacement frequency can be appropriately increased according to the actual situation.