In this paper, switch-type humidity sensors based on PAA (polyacrylic acid) and its copolymers were examined. The humidity sensing properties of these polymers, such as hysteresis, response time, water resistivity and long-term stability were investigated. The factors which influence the humidity sensors＇ inflection point were investigated experimentally as well. The results show that, the inflection point of the humidity sensors varies with the content of crosslinking agent and the sample composition. In particular, humidity sensors based on Sample E and B show good property and stability.