Abstract: An undersized humidity detecting system based on infrared spectrum absorption is introduced. A vertical-cavity surface-emitting laser is used as the light source and provides lasers at 1.854 μm which is a main absorption peak of water vapor. To cover all humidity condition, vapor density is detected via two different methods, which are switched by the system automatically depending on measure results. In low-density condition, the ratio of the second-harmonic to fundamental signal is used to eliminate the interference due to light power variation. A/D conversion with over-sampling and digital lock in amplification enhance the accuracy of signal processing, and with IEEE 802.11x protocol, automatic remote data acquisition is achieved.