Leaf Relative Water Content (LRWC) is a biophysical variable that quantitatively expresses water volume per leaf.LRWC is an important factor in determining the health and productivity of vegetation.It also reflects the plant ability of tolerance to drought stress.So it has become a very important phenotype indicator in plant drought stress resistance and water utilization efficiency studies.Conventional (destructive) methods such as comparing the fresh and dry weight of plant tissues is time consuming, labor-intensive, and destructive sampling.Moreover,it is not allow the instantaneous and continuous monitoring of the water content in living tissue.Currently, no destructive methods include: infrared spectrum detection, leaf capacitance detection,and leaf resistance detection.The detection method based on the spectroscopy is rapid and efficiency.According to water molecules absorption and transmittance infrared spectroscopy of leaf, the 980nm and 890nm are selected, in which the 980nm is the O-H absorption peak and 890nm is the reference wavelength.The detector will be developed for the digital signal measurement.Parameters could be calculated by the measuring signal such as Ratio Vegetation Index (RVI), Normalized Difference Vegetation Index (NDVI) and so on.The optical detector was developed for the leaves water content measurement in the field.It was based on transmission detection and included the active lighting, transmission light detection and signal processing, the infrared LED was used as active light source to product emission light at 890nm and 980nm.Photoelectric device was used to transform light absorbance into electric signal for further analyze and calculation.Through validation, relationship between LRWC and light absorbance can be constructed.Zigbee was applied for the data transition and a PDA was used to collect data and showed results.It provides a convenient method to measure LRWC in the field.