Abstract: Condensation water can accelerate weathering of stone relics. Previous methods for detecting condensation water in grottoes have some shortcomings, such as the complex equipment layout and limited monitoring range, and are unable to reflect the real-time condensation state, which restrict the effective prevention and control of condensation water in grottoes. In this paper, the infrared thermal imaging technology, which can be used to obtain continuous and large-scale observation of temperature on the wall of grottoes, and the microwave moisture measurement technology, which can be used to obtain rock moisture inside the wall, are both used to study the formation mechanism of condensation water. During the occurrence of a rainfall event, a wall in cave 19 of the Yungang Grottoes was monitored by the infrared thermal imaging for 100 hours. This set of data reveals the diurnal variation pattern of temperature before and during the rainfall event, and two periods beneficial for the formation of condensation water were identified by comparing with the air dew point temperature. The two periods are the condensation period before cooling which starts from 16 hours before rainfall to a sudden drop of temperature during rainfall and the condensation period after cooling which starts from the sudden drop of temperature to 12 hours after the end of rainfall. At the same time, the dynamic monitoring of the shallow rock moisture content for 56 hours was carried out with a microwave moisture meter. Although condensation water cannot be directly observed on the wall, the microwave moisture meter shows that water content in the rock increases, indicating that water has been preferentially condensed inside the rock. It is also found that the sudden drop of air temperature (from 18.5 °C to 15.6 °C in 1 hour) during the process of rainfall causes the condensation of water vapor in atmosphere, which causes a significant decline in the dew point temperature, and the condition of condensation water formation is not fulfilled. This phenomenon is of important guiding significance for the prevention and control of condensation water.