Abstract: In order to explore the reveal water state of plant, and research the plant canopy-air temperature difference and leaf thickness variation, the leaf temperature measuring instrument was developed, meanwhile, the YI-201020 leaf thickness precision instrument and leaf temperature measuring instrument were applied to monitor the leaf thickness and leaf temperature of chilies and peanuts at different stages, then the correlations between plant canopy-air temperature difference and leaf thickness after taking ambient temperature into consideration were analyzed. The experimental results showed that: the sensor linearity was 1%, the return error was 0.02 V, the sensitivity was 0.04 V/℃, the resolution was 0.3℃, the maximum indication error of instrument in the range of 0~90℃ was 0.27℃, and the expanded uncertainty was 0.41℃; the leaf thickness was in negative relationship with canopy-air temperature difference at every stage of the two plants, especially during the growth stage. The results in this paper provide a reference for the plant physics research.