Polymer layer capacitor humidity sensor is a sort of hew humidity detecting element of thin film capacitor pattern with high accuracy and rapid responce, which can be worked in all range of humidity and in the temperature range of + 40℃ to-40℃. This paper discribes chiefly the research of this sensor that includes its operational Principle, constitution, specifications and test conditions in fields.This sensor have been measured in laboratory by using standard equipments in the range of temperature down to-40℃ and tested at some weather stations for half to one years. It is proved that its specifications obtaind the level of the same type of sensor made of VAISALA Company in Finland.