Abstract: Based on the principle that electricity characteristic of capacitor was influenced, the method for measuring grain moisture content by dielectric loss tangent was put forward. The hardware structure of grain moisture measuring instrument was developed on the basis of AT89C52 single-chip computer. The method for signal processing and measuring was described in detail. Through demarcating sensor and corresponding regressive processing, the relationship between dielectric loss tangent and grain moisture content can be obtained. If only dielectric loss tangent of grain was measured, the measuring system can acquire grain moisture content by seeking table and calculating. The results of experiments show that it has high accuracy and good repeatability.