In view of the current situation that the Moistening Weather Phenomena can only be observed by man, an automatic monitoring method is designed. In the system of temperature, humidity sensors and online calibration data monitor based on STM32, Kelvin Four-terminal sensing and differential self-calibration are adopted. Air temperature, relative humidity and surface temperature of floor can be measured accurately. And dew-point temperature is calculated by inverse function of New-Magnus formula. Then "Moistening Weather" process can be monitored in real time. Experiments show that the measurement can prompt the exact time of generation and disappearance. Light warning is sent when trend angle A≤15°, moderate warning is sent when 15°＜A≤30° , and severe warning is sent when A＞30°. It is important reference for waterproofing measures. The design can provide reference for the development of particular weather phenomenon monitoring.