In meteorological observation services, artificial methods are usually used to determine the occurrence of dew and frost phenomena. Manual observation has strong subjectivity, fewer observing times and low efficiency and it can not fully and continuously reflect the characteristics of dew and frost process. This article describes a dew and frost automatic observation device. The device works by detecting changes in capacitance of capacitive sensors in high humidity and low humidity conditions and measuring frequency changes caused by the change in capacitance. Then the recognition software is used to judge occurrence of dew phenomenon and frost phenomenon combined with freezing temperature information and icing information in order to achieve the dew and frost automatic observation. Through a lot of dew and frost comparative tests, we found the automatic observation device can accurately identify the occurrence of dew and frost process and the device can be extended to the practical use.